res_pjsip: Move tenantid to end of ast_sip_endpoint

res_pjsip: Move tenantid to end of ast_sip_endpoint

The tenantid field was originally added to the ast_sip_endpoint
structure at the end of the AST_DECLARE_STRING_FIELDS block.  This
caused everything after it in the structure to move down in memory
and break ABI compatibility.  It's now at the end of the structure
as an AST_STRING_FIELD_EXTENDED.  Given the number of string fields
in the structure now, the initial string field allocation was
also increased from 64 to 128 bytes.

Resolves: #982
(cherry picked from commit 3dfac27ef2af798286d001f71335698b5fcb3777)