Feature: Allow headless apps to authenticate

[VallariAg]

Allow devices to authenticate to t-api through github.
This would be helpful to allow scripts to authenticate to t-api and schedule runs.

For auto-scheduling feature, this could allow ceph-build to authenticate to t-api to send build status updates.

Ref: https://docs.github.com/en/apps/oauth-apps/building-oauth-apps/authorizing-oauth-apps#device-flow

[VallariAg]

Update: we agreed on call that for auto-scheduling feature, we would go for some basic auth between our two services.