Automatic Social Account Merging

**Is your feature request related to a problem? Please describe.**
Automatic Social Account Merging does not occur by default, https://github.com/aws-amplify/amplify-cli/issues/4208 is the original report, but the author closed it after finding their own solution.

**Describe the solution you'd like**
Automatically merge social accounts based on one of two criteria:
- They both define the same email address in their social account metadata.( https://github.com/aws-amplify/amplify-cli/issues/4208 )
- Simultaneous Logins. (This was typically called something similar to "Social account linking" in most of the libraries and tools I've used.)

**Describe alternatives you've considered**
I could build this myself with a bunch of lambdas and other stuff.

**Additional context**
This sort of functionality is the default in the majority of authentication services I have used. It was genuinely shocking to discover this was not the case while I was searching through the repo's GithHub issues for a different problem and came across #4208. While I'm sure this is primarily a case of terrible default behaviour in the underlying Cognito service, there is a workaround, and from what I can tell, Amplify is trying to be a "we do the boilerplate stuff for you" type of tool, so handling this kind of workaround/boilerplate feels like something Amplify should be doing.