Unify bRPC third party support doc/example with the bazel WORKSPACE file

[Superskyyy]

Is your feature request related to a problem? (你需要的功能是否与某个问题有关?) https://github.com/apache/brpc/pull/1996

Describe the solution you'd like (描述你期望的解决方法) The mentioned PR enabled using bRPC as a third party dependency but it is inconvenient to maintain as a subset of the workspace (i.e., sync dependency updates for protobuf).

Describe alternatives you've considered (描述你想到的折衷方案) Let root WORKSPACE import the third party brpc_workspace.bzl.

Additional context/screenshots (更多上下文/截图)