Add elasticgraph-protobuf schema artifacts

[jwils]

Why

• continue the pluggable ingestion serializer proposal after pulling JSON Schema into its own gem
• revive the earlier protobuf prototype from Add elasticgraph-proto gem with stable proto field-number mappings #1056 on top of the new serializer extension points

What

• add a new elasticgraph-protobuf extension gem that generates schema.proto and proto_field_numbers.yaml
• keep schema.proto on public GraphQL field names while storing private name_in_index overrides in the sidecar mapping file
• emit the generated schema.proto artifact preamble with protobuf // comments so it remains valid proto syntax
• preserve field numbers across public renames via field.renamed_from and update repo wiring/docs/tests

References

• Feature Proposal: Support for Protobuf ingestion #1059
• Add elasticgraph-proto gem with stable proto field-number mappings #1056
• Pluggable ingestion serializers with json_ingestion #1079