Boards integration using GitHub app instead of PAT

Currently the way integrate-boards command works is it creates the connection using a GitHub PAT. This was done mainly because it was easier/quicker to implement. But it means that when that PAT expires all the Boards integrations will break.

Would be better to create the connection using the Boards GitHub App instead of a PAT. Like with pipelines we would expect the customer to pre-configure the app in the GitHub org before using the CLI. The process of wiring up the GH app in ADO might be a bit trickier than with Pipelines however, since I don't think we have the option of sharing a service connection here.