Source enum values from external proto enums and reference external enum types

[jwils]

Why

Some schemas already have canonical protobuf enum definitions. Generating duplicate enums in schema.proto makes Java consumers deal with parallel types, and maintaining enum value lists in two places invites drift.

What

Adds external_proto_enum on enum types, per the API inversion suggested in review — configuration lives on the enum type itself instead of a global hash imported from application code:

schema.enum_type "Currency" do |t|
  t.values "USD", "CAD"
  t.external_proto_enum MyApp::Protos::Currency,
    exclusions: [:UNKNOWN_DO_NOT_USE],
    name_transform: ->(name) { name.sub(/\ACURRENCY_/, "") }
end

• Commit 1 — value sourcing: the generated proto enum's values come from the proto enum class (anything exposing .enums), honoring exclusions, expected_extras, and name_transform keyword args. Multiple sources may be registered and must agree.
• Commit 2 — external references: passing proto: and import: goes further — schema.proto imports the named file and references the existing enum type instead of generating a local duplicate. A referenced enum must have exactly one option-free source whose values match the ElasticGraph enum's values, so transformed or curated enums stay generated locally.

Because the API lives on enum types and options are real keyword arguments, several things from the previous revision disappear entirely: the global-hash normalization and string/symbol key handling, the extension state field, and two validation error cases that are now impossible by construction (an external reference without a value source, and referencing a non-enum type).

Risk Assessment

Low — only affects the unreleased elasticgraph-proto_ingestion extension, and both features are opt-in with eager validation.

References

• Stacked on Support proto2 syntax and custom proto file headers #1305 (→ Pin protobuf field and enum value numbers in a sidecar artifact #1304 → Generate a protobuf schema artifact from ElasticGraph schemas #1080)
• Addresses Generate a protobuf schema artifact from ElasticGraph schemas #1080 (comment)
• Replaces Reference external proto enum types #1265, which was auto-closed when a rebase temporarily collapsed this branch into its base