x/build/cmd/relui: add the binary signing step

[cagedmantis]

Relui should include a step where binaries are signed without any additional human interaction.

• Add a GRPC server x/build/internal/relui/protos: add the GRPC service for signing release artifacts. #54303

@golang/release

[gopherbot]

Change https://go.dev/cl/422598 mentions this issue: internal/relui/sign: add signing server

[gopherbot]

Change https://go.dev/cl/424914 mentions this issue: cmd/relui: add the release GRPC server to the HTTP server

[gopherbot]

Change https://go.dev/cl/424915 mentions this issue: deploy: add Release GRPC server to build.golang.org

[gopherbot]

Change https://go.dev/cl/428057 mentions this issue: internal/relui/protos: update wire protocol

[gopherbot]

Change https://go.dev/cl/428056 mentions this issue: internal/relui/protos: reorder protos