[ML] Add some ML config classes to protocol library (#32502)

This commit adds four ML config classes to the X-Pack protocol
library used by the high level REST client.

(Other commits will add the remaining config classes, plus results
and stats classes.)

These classes:

- Are immutable
- Have little/no validation of field values beyond null checks
- Are convertible to and from X-Content, but NOT wire transportable
- Have lenient parsers to maximize compatibility across versions
- Have the same class names, member names and getter/setter names
  as the corresponding classes in X-Pack core to ease migration
  for transport client users
- Don't reproduce all the methods that do calculations or
  transformations that the the corresponding classes in X-Pack core
  have

x-pack/protocol/src/main/java/org/elasticsearch/protocol/xpack/ml/job/config/AnalysisLimits.java

@@ -0,0 +1,139 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.elasticsearch.protocol.xpack.ml.job.config;
+
+import org.elasticsearch.common.Nullable;
+import org.elasticsearch.common.ParseField;
+import org.elasticsearch.common.unit.ByteSizeValue;
+import org.elasticsearch.common.xcontent.ConstructingObjectParser;
+import org.elasticsearch.common.xcontent.ObjectParser;
+import org.elasticsearch.common.xcontent.ToXContentObject;
+import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.common.xcontent.XContentParser;
+
+import java.io.IOException;
+import java.util.Objects;
+
+/**
+ * Analysis limits for autodetect. In particular,
+ * this is a collection of parameters that allow limiting
+ * the resources used by the job.
+ */
+public class AnalysisLimits implements ToXContentObject {
+
+    /**
+     * Serialisation field names
+     */
+    public static final ParseField MODEL_MEMORY_LIMIT = new ParseField("model_memory_limit");
+    public static final ParseField CATEGORIZATION_EXAMPLES_LIMIT = new ParseField("categorization_examples_limit");
+
+    public static final ConstructingObjectParser<AnalysisLimits, Void> PARSER =
+        new ConstructingObjectParser<>("analysis_limits", true, a -> new AnalysisLimits((Long) a[0], (Long) a[1]));
+
+    static {
+        PARSER.declareField(ConstructingObjectParser.optionalConstructorArg(), p -> {
+            if (p.currentToken() == XContentParser.Token.VALUE_STRING) {
+                return ByteSizeValue.parseBytesSizeValue(p.text(), MODEL_MEMORY_LIMIT.getPreferredName()).getMb();
+            } else if (p.currentToken() == XContentParser.Token.VALUE_NUMBER) {
+                return p.longValue();
+            }
+            throw new IllegalArgumentException("Unsupported token [" + p.currentToken() + "]");
+        }, MODEL_MEMORY_LIMIT, ObjectParser.ValueType.VALUE);
+        PARSER.declareLong(ConstructingObjectParser.optionalConstructorArg(), CATEGORIZATION_EXAMPLES_LIMIT);
+    }
+
+    /**
+     * The model memory limit in MiBs.
+     * It is initialised to <code>null</code>, which implies that the server-side default will be used.
+     */
+    private final Long modelMemoryLimit;
+
+    /**
+     * It is initialised to <code>null</code>.
+     * A value of <code>null</code> will result in the server-side default being used.
+     */
+    private final Long categorizationExamplesLimit;
+
+    public AnalysisLimits(Long categorizationExamplesLimit) {
+        this(null, categorizationExamplesLimit);
+    }
+
+    public AnalysisLimits(Long modelMemoryLimit, Long categorizationExamplesLimit) {
+        this.modelMemoryLimit = modelMemoryLimit;
+        this.categorizationExamplesLimit = categorizationExamplesLimit;
+    }
+
+    /**
+     * Maximum size of the model in MB before the anomaly detector
+     * will drop new samples to prevent the model using any more
+     * memory.
+     *
+     * @return The set memory limit or <code>null</code> if not set
+     */
+    @Nullable
+    public Long getModelMemoryLimit() {
+        return modelMemoryLimit;
+    }
+
+    /**
+     * Gets the limit to the number of examples that are stored per category
+     *
+     * @return the limit or <code>null</code> if not set
+     */
+    @Nullable
+    public Long getCategorizationExamplesLimit() {
+        return categorizationExamplesLimit;
+    }
+
+    @Override
+    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+        builder.startObject();
+        if (modelMemoryLimit != null) {
+            builder.field(MODEL_MEMORY_LIMIT.getPreferredName(), modelMemoryLimit + "mb");
+        }
+        if (categorizationExamplesLimit != null) {
+            builder.field(CATEGORIZATION_EXAMPLES_LIMIT.getPreferredName(), categorizationExamplesLimit);
+        }
+        builder.endObject();
+        return builder;
+    }
+
+    /**
+     * Overridden equality test
+     */
+    @Override
+    public boolean equals(Object other) {
+        if (this == other) {
+            return true;
+        }
+
+        if (other instanceof AnalysisLimits == false) {
+            return false;
+        }
+
+        AnalysisLimits that = (AnalysisLimits) other;
+        return Objects.equals(this.modelMemoryLimit, that.modelMemoryLimit) &&
+                Objects.equals(this.categorizationExamplesLimit, that.categorizationExamplesLimit);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(modelMemoryLimit, categorizationExamplesLimit);
+    }
+}