Show region picker for private-link create when --region is unset

[shreyasj-livekit]

Summary

• Factors the region-resolution UX out of agent create into a shared resolveRegion helper in cmd/lk/agent.go.
• Reuses the helper in private-link create so that command now presents the same interactive picker of available regions (pulled from available_regions client settings) when --region isn't passed.
• Relaxes --region on private-link create from Required: true to optional so the picker can kick in; non-interactive invocations still fail loudly with the list of available regions instead of silently defaulting.

What it looks like

lk) drshrey@Shreyass-MBP livekit-cli % ./bin/lk agent private-link create --name test-pl --port 6379 --endpoint foobar
Using default project [private-cloud-agents-project3]
┃ Select region for private link                                                            
┃   us-west                                                                                 
┃   us-central                                                                              
┃ > ca-central