Add ability to bypass automatic ECA checks entirely for select GH repos

This is in response to https://gitlab.eclipse.org/eclipsefdn/infrazilla/-/issues/2253#note_4969315, which needs a way of disabling ECA checks for some repositories. In this case, the least brittle way of accomplishing this goal would be a configuration item in the API, documentation about the new feature, and a small update to the validation logic.