res_pjsip.c: Support endpoints with domain info in username

In multidomain environments, it is desirable to create
PJSIP endpoints with the domain info in the endpoint name
in pjsip_endpoint.conf.  This resulted in an error with
registrations, NOTIFY, and OPTIONS packet generation.

This commit will detect if there is an @ in the endpoint
identifier and generate the URI accordingly so NOTIFY and
OPTIONS From headers will generate correctly.

ASTERISK-28393

Change-Id: I96f8d01dfdd5573ba7a28299e46271dd4210b619