Zero Downtime Migration with Temporal Cloud Automation

What this post added

Introduces new migration tooling for transitioning from self-hosted Temporal to Temporal Cloud. The tooling automates the process with zero downtime for workflows, utilizing a migration server and proxies. It outlines a step-by-step guide including preparation, initiation via `StartMigrationRequest`, progress monitoring via `GetMigrationResponse`, and handover/finalization/abortion via `HandoverNamespaceRequest`, `ConfirmMigrationRequest`, and `AbortMigrationRequest` APIs. This feature is currently in pre-release and supports only self-hosted to Cloud migrations.