Release App-Config extension

[JasonYeMSFT]

• [x] Add proxy, User Agent support
• [x] Azure China, Azure US Gov support
• [x] Finish test pass
• [x] Create App Config Extension GitHub repository
• [ ] Update doc page for direct links added by extensions
• [x] Update the extension label to say (preview)
• [x] Update the feedback url
• [x] Update landing page
• [x] Upload bits to our extension container

[JasonYeMSFT]

The extension has been released at https://github.com/microsoft/azure-app-configuration-se-extension. The remaining work will need to be done in 1.26.0.

[JasonYeMSFT]

All the work has been done.