protoc-gen-go-drpc: generate HTTP gateway routes from proto annotations

[shubhamdhama]

Read google.api.http annotations from proto method options and emit
DRPC<Service>GatewayRoutes functions in the generated *_drpc.pb.go
files. Each function returns a []drpc.HTTPRoute containing the HTTP
method, path, and a reference to the corresponding RPC client method.

This lets consumers (like CockroachDB's DRPC gateway) register HTTP
routes from generated data instead of maintaining manual route tables
that drift out of sync with proto definitions. Streaming RPCs and
methods without HTTP annotations are skipped.

[Copilot]

Pull request overview

Adds HTTP gateway route metadata generation to protoc-gen-go-drpc by reading google.api.http method annotations and emitting per-service DRPC<Service>GatewayRoutes helpers that return []drpc.HTTPRoute for consumers to register routes programmatically.

Changes:

• Introduces drpc.HTTPRoute as a shared struct for generated HTTP route metadata.
• Extends protoc-gen-go-drpc to read google.api.http annotations and generate DRPC<Service>GatewayRoutes functions (skipping streaming RPCs / unannotated methods).
• Adds google.golang.org/genproto/googleapis/api dependency for HTTP annotations.

Reviewed changes

Copilot reviewed 3 out of 5 changed files in this pull request and generated 1 comment.

File	Description
go.mod	Adds genproto API module dependency required to read google.api.http annotations.
go.sum	Updates sums for newly added genproto modules.
drpc.go	Adds exported HTTPRoute struct used by generated gateway route helpers.
cmd/protoc-gen-go-drpc/main.go	Implements annotation parsing and emits DRPC<Service>GatewayRoutes with per-binding routes.

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[cthumuluru-crdb]

Can you include an example of generated code in the comment?

[shubhamdhama]

@cthumuluru-crdb done

[cthumuluru-crdb]

LGTM! Can you also add a sample generated code as a reference before you merge these changes?