ccl/sqlproxyccl: add postgres interceptors for message forwarding

Informs #76000.

This commit implements postgres interceptors, namely FrontendInterceptor and
BackendInterceptor, as described in the sqlproxy connection migration RFC.
These interceptors will be used as building blocks for the forwarder component
that we will be adding in a later PR. Since the forwarder component has not
been added, a simple proxy test (i.e. TestSimpleProxy) has been added to
illustrate how the frontend and backend interceptors can be used within the
proxy.

Release note: None

pkg/BUILD.bazel

@@ -41,6 +41,7 @@ ALL_TESTS = [
     "//pkg/ccl/spanconfigccl/spanconfigsqlwatcherccl:spanconfigsqlwatcherccl_test",
     "//pkg/ccl/sqlproxyccl/denylist:denylist_test",
     "//pkg/ccl/sqlproxyccl/idle:idle_test",
+    "//pkg/ccl/sqlproxyccl/interceptor:interceptor_test",
     "//pkg/ccl/sqlproxyccl/tenant:tenant_test",
     "//pkg/ccl/sqlproxyccl/throttler:throttler_test",
     "//pkg/ccl/sqlproxyccl:sqlproxyccl_test",

pkg/ccl/sqlproxyccl/interceptor/BUILD.bazel

@@ -0,0 +1,26 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "interceptor",
+    srcs = ["interceptor.go"],
+    importpath = "github.com/cockroachdb/cockroach/pkg/ccl/sqlproxyccl/interceptor",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//pkg/sql/pgwire/pgwirebase",
+        "@com_github_cockroachdb_errors//:errors",
+        "@com_github_jackc_pgproto3_v2//:pgproto3",
+    ],
+)
+
+go_test(
+    name = "interceptor_test",
+    srcs = ["interceptor_test.go"],
+    embed = [":interceptor"],
+    deps = [
+        "//pkg/sql/pgwire/pgwirebase",
+        "//pkg/util/leaktest",
+        "@com_github_cockroachdb_errors//:errors",
+        "@com_github_jackc_pgproto3_v2//:pgproto3",
+        "@com_github_stretchr_testify//require",
+    ],
+)