Enable application to provide its own external resolver implementation

[riza]

Current PJSIP resolver lacks these functionality:

• NAPTR support
• full IPv6 support
• unspecified SRV lookups

This ticket will enable application define its own resolver implementation and set it to
be used instead of the method built in. Application can do this by by defining the callback in
pjsip_ext_resolver and pass it to pjsip_endpt_set_ext_resolver() function.

Thanks to Joshua Colp for the suggestion and initial patch.

[riza]

In 5055:

Re #1838: Enable application to provide its own external resolver implementation.