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

[jjgriff93]

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.