res_pjsip_exten_state: Check if body generator is available.

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