Add mutability of rest of document in Ingest-ForeachProcessor sub processors

[talevy]

Spoke with @martijnvg and @BigFunger about some strange behavior around mutated state within sub-processors within ForEach. Realization was that we expose top-level document fields, but silently ignore mutations to them. This is a really confusing behavior.

Currently, the foreach processor makes it possible to read all other fields in the ingest document besides _value. It does not enable mutability of these fields.

This test passes that attempts to append to a top-level field passes. The append does not actually do anything since it operates on a copy. This should be fixed so that the original documents reflects modifications outside of _value.

    public void testExecute() throws Exception {
        List<Object> values = new ArrayList<>();
        values.add("string");
        values.add(1);
        values.add(null);
        IngestDocument ingestDocument = new IngestDocument(
            "_index", "_type", "_id", null, null, null, null, Collections.singletonMap("values", values)
        );

        TemplateService ts = TestTemplateService.instance();

        ForEachProcessor processor = new ForEachProcessor(
            "_tag", "values", new CompoundProcessor(false,
            Collections.singletonList(new UppercaseProcessor("_tag_upper", "_value")),
            Collections.singletonList(new AppendProcessor("_tag", ts.compile("errors"), (model) -> (Collections.singletonList("added"))))
        ));
        processor.execute(ingestDocument);

        List<String> result = ingestDocument.getFieldValue("values", List.class);
        assertThat(result.get(0), equalTo("STRING"));
        assertThat(result.get(1), equalTo(1));
        assertThat(result.get(2), equalTo(null));
    }