Maintenance mode (Support Server Error 5xx)

**Is your feature request related to a problem? Please describe.**
Sometimes we want to stop serving our web app temporarily for database migration, updating backends, etc. Currently there does not seem to be such feature provided.

**Describe the solution you'd like**
We want a capability of Amplify Console to switch their webapp to Maintenance mode, where a maintenance page is served with specific status code (e.g. 503 Service Unavailable).

**Additional context**
Alternatively, we can achieve a similar request by creating another branch that has the maintenance page and switch the branch of the custom domain. However, this workaround does not work if we want to serve the page with status code 503. So we will need to use other services such as ALB or CF (with Lambda@Edge) + S3 to serve an HTML file with the specific status code, which is costly.

No source file, test, or entry point is named in the issue. Start by mapping Amplify Console's hosting and custom-domain behavior, then define how a maintenance-mode control would serve the maintenance page with HTTP 503; done means the capability and its status-code behavior are clearly specified.

Written by the indexing model from the issue text.