[Release plan] allow service to specify a latest preview or a stable version in main.tsp for SDK releases

Currently, when a user initiates a release plan and triggers SDK generation, SDKs are produced only for the latest version by default. However, some services require support for both GA and preview versions simultaneously. Without the ability to specify an alternative version, SDK language engineers must manually customize code generation and re-initiate the generation, making self-service functionality unattainable.

Start with the release plan flow that triggers SDK generation and inspect how main.tsp is read. Trace where the latest version is selected, then verify that a service can request GA or preview versions and that generation supports the requested versions without manual customization.

Written by the indexing model from the issue text.