[coro_rpc][fix] keep protocol compatible/use context<T>::tag() instead o…

[poor-circle]

…f set_tag&get_tag/use coro_rpc::err_code instead of coro_rpc::errc

What is changing

1. Closes Why not try to keep the RPC protocol compatible #554 to keep compatible of rpc protocol. Now user's error code will be put in payload not in rpc_head.
2. use std::any& context<T>::tag() instead of set_tag/get_tag
3. the error type now change to coro_rpc::err_code to help error handling like:

  auto ec= client.connect();
  if (ec) {
     //....
  }

4. add example for context<T>::response_error & context<T>::tag

[qicosmos]

LGTM