Add `RDS_CMSG_RXPATH_LATENCY` to `socket`

Feature or enhancement

Proposal:

This issue suggests adding new RDS constants.

s = /* a RDS type socket */
msg, ancdata, flags, addr = s.recvmsg(2048, socket.CMSG_LEN(5 * 128))
for cmsg_level, cmsg_type, cmsg_data in ancdata:
    if cmsg_type == RDS_CMSG_RXPATH_LATENCY:
        # some processing

It‘s used to query the rx-path(that's receive) latency of RDS. Usually used to optimize RDS communication, such as controlling the number of connections and other strategies

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-127384

首先檢查 socket 模組對 RDS 輔助資料的處理，以及提案中描述的 recvmsg 路徑。檢查連結的 PR gh-127384，了解目前的實作和預期的測試涵蓋；完成意味著常數 RDS_CMSG_RXPATH_LATENCY 已公開，並且可以與所示的輔助資料流搭配使用。