Preserve custom FieldOptions extensions when loading schema metadata from JSON

[pengpeng-lu]

Summary

• CommandUtil.loadRecordMetaDataFromJson parses downloaded schema metadata JSON via
  JsonFormat.parser().ignoringUnknownFields(). JsonFormat (protobuf-java-util) cannot parse
  proto2 extensions under any configuration, so any custom FieldOptions extension in the
  JSON (e.g. a per-field marker used to distinguish multiple RecordTypeUnion generations of the
  same record type) was silently dropped during parsing.
• Once dropped, two different generations of the same record type reduce to identical (empty)
  FieldOptions, and RecordLayerTable.Builder.addGeneration's dedup check throws
  "Duplicated options for different generations of Table X" as soon as a schema has more than
  one generation of the same record type in its RecordTypeUnion.
• Adds CommandUtil.restoreFieldOptionExtensions (+ helpers): after the normal JsonFormat merge
  and dependency resolution, it walks the raw JSON tree, finds any FieldOptions extensions
  declared by the already-resolved proto dependencies, and copies their values back in via
  protobuf's reflective DynamicMessage/setField API, which — unlike JsonFormat — does
  support extensions.

Test

• Added custom_field_option.proto, a small self-contained proto2 FieldOptions extension used
  only by this test.
• Added field-options-extension-metadata.json + field-options-extension.yamsql: a
  RecordTypeUnion with two fields referencing the same record type, each carrying a distinct
  extension value; registered in YamlIntegrationTests as fieldOptionsExtensionTest.
• Manually confirmed the regression is caught: temporarily disabling the fix reproduces
  "Duplicated options for different generations of Table MyRecord"; re-enabling it passes.