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

**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)**

Start by comparing the root WORKSPACE with brpc_workspace.bzl and reviewing the third-party support documentation/example introduced by PR #1996. Check how protobuf and other dependency updates are represented in each location. Done means the root WORKSPACE imports brpc_workspace.bzl and the documentation/example matches that single maintained setup.

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