When starting the extension state publishers, check if the requested
message body generator is available.  If not available give error message
and skip starting that publisher.

* res_pjsip_pubsub.c: Create new API if type/subtype generator
registered.

* res_pjsip_exten_state.c: Use new body generator API for validation.

ASTERISK-25922

Change-Id: I4ad69200666e3cc909d4619e3c81042d7f9db25c