Export EventNewInfo in public API

[eschmidbauer]

Export EventNewInfo from sip_ua public API

Summary

• Expose EventNewInfo via the public sip_ua.dart export list so downstream apps can subscribe to SIP INFO events without implementation_imports.

Why

• SIP INFO is delivered internally via EventNewInfo, but the type isn’t publicly exported.
• Apps currently must import lib/src to listen for INFO, which triggers implementation_imports warnings and is fragile.

Changes

• Add a public export for EventNewInfo in lib/sip_ua.dart.

Testing

• Not applicable; API export only.

Notes

• This is backwards-compatible and does not change runtime behavior; it only exposes an existing event type.

[eschmidbauer]

needs more work. will open another PR shortly