res_pjsip: Add 'ip' as a valid option to 'identify_by' on endpoint.

When the identify_by option on an endpoint is set to ip it will
only be identified using the res_pjsip_endpoint_identifier_ip module.
This ensures that it is not mistakenly matched using the username of
the From header. To ensure behavior has not changed the default has
been changed to "username,ip" for the identify_by option.

ASTERISK-27206

Change-Id: I2170b86a7f7e221b4f00bf14aa1ef1ac5b050bbd