feat: add diarize attribute to Colibri2Endpoint

[bgrozev]

Adds a per-endpoint diarize boolean attribute to the COLIBRI v2 Colibri2Endpoint (modeled like the existing stats-id / muc-role attributes, not a <capability> — this is an instruction to the bridge, not a "supports" advertisement). getDiarize() returns a nullable Boolean (absent = null); Builder.setDiarize(boolean) sets it.

Part of a multi-repo change enabling per-endpoint speaker diarization (jicofo signals it here, the videobridge reads it and sets it on the transcriber start event). The four Maven repos share the branch diarization-endpoint-control so CI builds them together; no dependency version bumps are included (those follow after the upstream repos are released).

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Related PRs (per-endpoint speaker diarization, multi-repo):

• jitsi-meet: feat(transcription): add diarize flag advertised as a participant property jitsi-meet#17614
• jitsi-xmpp-extensions: feat: add diarize attribute to Colibri2Endpoint #145
• jicoco: feat: add diarize field to mediajson Start event jicoco#238
• jicofo: feat: signal per-endpoint diarize to the bridge via Colibri2 jicofo#1291
• jitsi-videobridge: feat: set per-endpoint diarize on the transcriber start event jitsi-videobridge#2431
• opus-transcriber-proxy: feat: per-endpoint diarization via the start event opus-transcriber-proxy#106