Experimental gRPC server

[finnegancarroll]

Description

A gRPC server implemented in the transport-netty4 module as a network plugin. This server reuses existing Netty-4 dependencies and worker groups. The gRPC server interface as well as service definitions are defined in the server module under org/opensearch/grpc/. Services are injected in Node.java through the GrpcServiceRegistry.

All protobuf files are located in server/src/main/proto/ with the following loose convention:
/org/opensearch - Hand rolled proto files with package name mirroring corresponding java definitions.
/services - Service definitions.
/spec - Auto generated protobuf from the opensearch-api-spec. Source. More info - 1 2.

Feature flag: opensearch.experimental.feature.grpc.enabled
Note: Feature flag enabled by default for QOL.

Testing cheatsheet (grpcurl)

./gradlew run
grpcurl -plaintext localhost:9400 list
grpcurl -plaintext localhost:9400 opensearch.proto.services.SearchService
grpcurl -plaintext localhost:9400 opensearch.proto.services.DocumentService
grpcurl -plaintext localhost:9400 opensearch.proto.services.NodesInfoService

Considerations:

• Netty4 shaded vs gRPC/HTTP sharing Netty dependencies
• TLS/Security plugin

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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