Expose registrator accessor for runtime REGISTER customization

[mechtech-mind]

What this PR does

This PR exposes a safe accessor for the internal Registrator instance so that
applications can modify REGISTER / UNREGISTER extra Contact parameters at runtime
without forking the library.

Why this is needed

Some applications (e.g. RFC8599 / push / call correlation use cases) need to add
dynamic Contact URI parameters at the moment of REGISTER, such as a runtime
UUID or call identifier.

Previously this was not possible without forking, because:

• UA._registrator was private
• SIPUAHelper had no way to reach the active registrator

Changes

• Expose registratorInstance getter in UA
• Expose registrator getter in SIPUAHelper
• No behavior change, no breaking API changes

Usage example

sipHelper.registrator?.setExtraContactParams({
  'calluuid': uuid,
});
sipHelper.register();

Notes

• Fully backward compatible
• No changes to registration flow
• Minimal surface-area change (2 files, 3 LOC)
• This is an advanced-use feature intended for users familiar with the library
  internals, as the underlying capability already existed but was not exposed via
  the public plugin API.

Thanks!

Also feedback on additional test coverage or documentation improvements is welcome.

[mechtech-mind]

Hi @cloudwebrtc 👋

Just a gentle follow-up on this PR when you have time.
This is a small, backward-compatible change that exposes an existing internal
capability for advanced use cases (runtime REGISTER Contact customization).

All checks are passing, and I’m happy to make any adjustments if needed.
Thanks for maintaining the project!