iproto: send default authentication method in reply to IPROTO_ID

Part of #7989

@TarantoolBot document
Title: Document IPROTO_AUTH_TYPE - new key of IPROTO_ID

A new key was added to the `IPROTO_ID` response:
name `IPROTO_AUTH_TYPE`, code 0x5b, value type `MP_STR`.

Its value equals the name of the authentication method that is
currently used on the server for generating user authentication
data from a password `box.schema.passwd()`. It's configured with
`box.cfg.auth_type`.

In Community Edition, the only authentication method currently
available is 'chap-sha1'. Other authentication methods may be used
in Enterprise Edition.

A client can use the received value as the authentication method
that should be used by default, in case it isn't specified explicitly
along with the user credentials.