* Install `buf` instead of `protoc`
* Created `buf.yaml` and `buf.gen.yaml` files in the two proto directories to control how `buf` generates/lints proto code.
* Invoke `buf` instead of `protoc`
* Added a `proto-format` make target.
* Committed the reformatted proto files.
* Added a `proto-lint` make target.
* Integrated proto linting with CI
* Fixed tons of proto linter warnings.
* Got rid of deprecated builtin protoc-gen-go grpc plugin usage. Moved to direct usage of protoc-gen-go-grpc.
* Unified all proto directories / go packages around using pb prefixes but ensuring all proto packages do not have the prefix.
Introduces a gRPC endpoint for signing Connect leaf certificates. It's also
the first of the public gRPC endpoints to perform leader-forwarding, so
establishes the pattern of forwarding over the multiplexed internal RPC port.
Adds a new gRPC streaming endpoint (WatchRoots) that dataplane clients will
use to fetch the current list of active Connect CA roots and receive new
lists whenever the roots are rotated.
