Reorganize the ast_sip_endpoint structure into substructures.

(closes issue ASTERISK-22135)
reported by Matt Jordan

Review: https://reviewboard.asterisk.org/r/2707



git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@395748 f38db490-d61c-443f-a65b-d21fe96a405b