feat(widget): Support RTC transports discovery action

[BillCarsonFr]

Draft because depends on #6789

Implements MSC4515

• I've documented the public API changes in the appropriate changelog files (see Writing changelog entries).
• This PR was made with the help of AI.

Signed-off-by:

[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing valere/rtc/widget_transport_discovery (33d1892) with valere/rtc/msc4143_rtc_discovery (4845b3f)}