enhance: Address schema in webrtc-call-handling

[teikuran]

Expanded the description for Subscriber address to include supported formats and usage of anonymous SIP URI.

What type of PR is this?

• enhancement/feature

What this PR does / why we need it:

This PR extends the Address schema to support anonymous call origination.

Main change — anonymous call origination:
The reserved value sip:anonymous@anonymous.invalid, as defined in RFC 3323, is added to the Address schema. When this value is set as the originatorAddress, the session is handled as an anonymous call origination and the originator's identity is not presented to the receiver. The value is restricted to originatorAddress and must not be used as receiverAddress.

Supplementary change — home local numbers in the pattern:
While updating the pattern, support for home local numbers (3GPP TS 24.229 subclause 5.1.2A.1.5) has been added, in both tel URI form and SIP URI form.

Which issue(s) this PR fixes:

Fixes #177

Special notes for reviewers:

The following formats are supported:

• A TEL URI :

  • an international format; or
  • a home local number (as defined in 3GPP TS 24.229 subclause 5.1.2A.1.5) qualified by a phone-context parameter identifying the home network numbering plan.

• A SIP URI

  • SIP URI with a generic user part;
  • SIP URI with a home local number (user=phone). The user part MAY contain a home local number qualified by a phone-context parameter, in which case the user=phone URI parameter SHALL be appended;
  • an emergency service URN; or
  • anonymous SIP URI
    • The reserved value sip:anonymous@anonymous.invalid (as defined in RFC 3323) MAY be used to indicate that the calling party identity is withheld within the trust domain. When sip:anonymous@anonymous.invalid is set as the originatorAddress, the session SHALL be handled as an anonymous call origination. This value SHALL NOT be used as the receiverAddress.

Changelog input

 release-note - webrtc-call-handling
- Added reserved value sip:anonymous@anonymous.invalid (RFC 3323) to Address schema to support anonymous call origination
- Extended Address pattern to support home local numbers (3GPP TS 24.229) in tel URI and SIP URI formats