Support for tel: URI in PJSUA-LIB (thanks Johan Lantz for the suggestion)

[bennylp]

While PJSIP has been supporting tel: URI for a long time, support for this is currently disabled in PJSUA-LIB, mainly for cosmetic reason.

This ticket enables tel: URI support in PJSUA-LIB, according to the following specification:

1. tel: URI in account ID shall be supported, and the tel: URI shall be used as the URI in From header for all requests originated from that account, such as :
   • client registration
   • outgoing calls (including re-INVITE and UPDATE)
   • outgoing REFER
   • outgoing MESSAGE
   • presence PUBLISH
   • outgoing SUBSCRIBE
   • and any other requests
2. shall be able to send requests to tel: URI from an account, as long as route set is configured for the account. This includes, but not limited to:
   • outgoing calls (including re-INVITE and UPDATE)
   • outgoing REFER
   • outgoing UPDATE
   • outgoing MESSAGE
   • outgoing SUBSCRIBE
   • and any other requests (although sending REGISTER and PUBLISH to a tel: URI is probably not appropriate)
3. shall be able to receive incoming requests with tel: URI in Request URI and/or To header of:
   • incoming calls
   • incoming MESSAGE
   • incoming REFER
   • incoming SUBSCRIBE
   • and any other requests

The unit tests shall be updated with above test scenarios.