Support atypical SRV naming for TLS

[troutkilroy]

Describe the feature

Support SRV lookup for TLS SIP domains that use something other than a _sips._tcp prefix namely:

_sips._tls _sip._tls

Describe alternatives you've considered

Implementing our own SRV resolution for TLS transport instead of specifying a nameserver in PJSIP.

Additional context

We've need to support SIP providers that prefix TLS SRV records as _sips._tls or _sip._tls. PJSIP always assumes _sips._tcp. is the TLS domain prefix for SRV lookup.