GitHub Advisories: Test Connection Button

### Current Behavior:
GitHub Advisory mirroring is refreshed daily, or upon restart of the Dependency-Track instance. #1542 is currently scheduled for milestone 4.6 and will allow the user to configure GitHub Advisory mirroring to refresh (say) every 6 hours. That is great... but it still means that there will be a wait of 6 hours to find out if the PAT was valid (perhaps copy and paste of the token accidentally chopped off a character) AND one will only find out this from the logs, not from the UI.

### Proposed Behavior:
Implement ability to test the connection via a button. Thus similar to #265 for OSSI.