iproto: introduce IPROTO_ID request

The new request can be used by a client to let the server know about
supported IPROTO protocol features. The request body contains two
fields: IPROTO protocol version and supported IPROTO protocol features.
In reply to a IPROTO_ID request, the server sends its own protocol
version and supported features.

Currently, the actual protocol version is 1 and there are two features
defined which are always set - streams and transactions.

Part of #6253

@TarantoolBot document
Title: Document IPROTO_ID request

The new request code is 73. It takes a map in the body with the
following keys:
 - IPROTO_VERSION (0x54) - protocol version (unsigned).
 - IPROTO_FEATURES (0x55) - array of protocol feature ids (unsigned).

A client (connector) can send this request to let the server know about
the protocol version and features it supports. The server may enable or
disable certain functionality basing on the features supported by the
client. In reply to the request, the server sends an IPROTO_OK response,
in the body of which it reports its own protocol version and features
(the format of the response body is the same as the request body).
The request doesn't need authentication to pass.

Currently, the actual protocol version is 1 and there are two features
defined:
 - IPROTO_FEATURE_STREAMS - streams support (IPROTO_STREAM_ID header
   key), id = 0.
 - IPROTO_FEATURE_TRANSACTIONS - transactions support (IPROTO_BEGIN,
   IPROTO_COMMIT, IPROTO_ROLLBACK commands), id = 1.