Support for Multi-data-center Amplify

As we learnt today, it would be good to have our apps work across multiple data-centres when there are outages at specific data-centres. It would be good to have this supported out of the box in amplify.

amplify datacenter add
- which datacenter would you like to setup
- how would you like to sync data
- how would you like to deploy (all at once, or by data centre)
- how would you like to switchover (manual, site-down hot-swap , load-balanced)
** here are some warnings about things that might not work reliably **

amplify deploy (does a deploy to all data centres)
amplify deploy us-east-1 (deploys to individual data centre)

Start by reviewing the Amplify CLI entry points for `amplify datacenter add` and `amplify deploy`. Clarify how data synchronization, deployment scope, switchover modes, and reliability warnings should work across data centres. Done means the multi-data-centre workflow and both deployment modes are defined and supported consistently.

Written by the indexing model from the issue text.