PR Bot functionality for validating deployment configuration/custom templates before merge

For scenarios where configuration changes (i.e. updating a network address space) and custom template additions are made and PR'd into a user's deployment repo, it would be useful to have an out-of-the-box PR comment bot to run an ephemeral build (deployment of the TRE) before PRs are merged to a main branch and update the user's persistent environment, much like we do in the AzureTRE repo itself.

This would help users to validate any changes to configuration, custom templates etc. and test a deployment before deploying to a persistent env.

Would welcome some thoughts and discussion on this and happy to implement if there's agreement.

Start by reviewing the existing PR bot and deployment workflow in the AzureTRE repository, which the issue identifies as the model. Define how a deployment repository would trigger an ephemeral TRE build for configuration or custom-template changes and report the result as a PR comment; done means the workflow is agreed and validates changes before merge without updating the persistent environment.