Option to setup a frontend redirection to maintenance page during build process

### Is this related to a new or existing Amplify category?

api, hosting

### Describe the feature you'd like to request

As an amplify developer/administrator, I would like an option for the build process allowing to redirect all users currently connected to the (react) frontend while the process is ongoing.
So that users would see something like that:

Not sure if this makes sense, but I think it might be a general need.

### Describe the solution you'd like

As described aws-amplify/amplify-category-api#180, I am trying to figure out the steps required when changing graphql schema and related data migration.
As part of that, we thought that it would make sense to block users from making any data change (or at least reduce them) while the build process is ongoing.
A way to do this, is to show a maintenance page that doesn't allow users to take any action.

Many platforms have some strategy for the maintenance of the system. If we want to "generalize" the feature request, the question is: does it make sense for amplify to provide features in this area?
I assume anyone with a system in production will face similar issues, so maybe a common solution could be useful.

### Describe alternatives you've considered

Custom implementation.
I have described it a bit there (it's just a draft for now): aws-amplify/amplify-category-api#180

Start by reviewing the Amplify Hosting build process and the related aws-amplify/amplify-category-api#180 discussion, including the described React frontend migration scenario. Determine whether maintenance-page redirection belongs in Amplify Hosting and define the supported configuration and behavior. Done means the scope is agreed and users are redirected to a non-mutating maintenance page while builds run.

Written by the indexing model from the issue text.