TS API to get origin socket fd

Currently, there's an API to get the client socket's fd TSHttpTxnClientFdGet and TSHttpSsnClientFdGet. This API is used by the tcpinfo plugin to get tcp stats for the client side socket. This jira proposes to add the corresponding API for server socket fd.

TSReturnCode TSHttpTxnServerFdGet(TSHttpTxn txnp, int *fdp);

For the server side, it should never be possible to have a Ssn without Txn, a Ssn wrapper API (e.g.TSHttpSsnServerFdGet) is not required

Note that the API will return TS_ERROR for a txn that returns a cached response with the fd set to -1