Add --cloud-region support to agent private-link create

[shreyasj-livekit]

Summary

• add optional --cloud-region flag to lk agent private-link create
• update command description to include Azure alias + Azure Resource ID endpoint formats
• pass cloud_region through CreatePrivateLinkRequest
• bump github.com/livekit/protocol to include cloud_region field

Validation

• go test ./cmd/lk -run TestBuildCreatePrivateLinkRequest_HappyPath -count=1

Depends On

• Add cloud_region to CreatePrivateLinkRequest protocol#1498