While prost-build already exposes protobuf message types as structs, we
actually need tonic-build too, to be able to get traits for all the RPC
services defined in the proto files.
Change-Id: I7f4c08454bf0d280d577975c7cdae13ccc2d933b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7320
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>