#7797515 Add Authorize GitHub card

Closes #70376
Related D99604-code

You can now authorise/connect GitHub via the /hosting-config/:site page in the same way as via /marketing/connections/:site.

Proposed Changes

• Add a GitHub card for authorising WordPress to access GitHub.
• Conditionally render the card according to the current state. A placeholder view from #70378 is rendered if you are already connected.

Before authorising:

After authorising (just a placeholder):

Testing Instructions

• Checkout this branch of use Calypso Live
• Follow the test plan at D99604-code, but don’t connect via the marketing page
• Go to /hosting-config/:site and click “Authorize”
• Authorize WordPress in the popup window
• Verify you see the “Connect Branch” UI after the popup closes.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the “[Status] Needs Privacy Updates” label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?

Related to #