Promote google/protobuf/api.proto as a runtime proto

[oldergod]

this includes its dependencies. This is useful for people who want to define a path on their RPCs without having to load protobuf on their proto path manually.

[JakeWharton]

I don't know what this means but if you want a green checkmark I am your guy

[oldergod]

Protobuf exposes option so that someone can embed the path of an RPC in the proto schema like so

  rpc MyRpc (MyRequest) returns (MyResponse) {
    option (google.api.http) = {
      post: "/cash/my-path"
    };
  }

Before this PR, people wanting to use this option would have to manually add google/protobuf/api.proto to their path. After the PR, Wire does it by default.