Add More/Custom Oauth Providers

**Is your feature request related to a problem? Please describe.**
The current list of available authentication providers is very limited. There are also no guides on how to setup a custom provider manually, and there isn't many 3rd party resources on it to tie custom Oauth providers in with Amplify specifically.

**Describe the solution you'd like**
More common providers setup, such as Twitch, Twitter, GitHub, Apple, Game Center, Microsoft, etc. Also allow easy linkage with multiple providers, especially for gaming related services, you will probably add at least Twitch, Microsoft for Xbox, Steam, Apple, Game Center.

If adding additional providers isn't in the roadmap, maybe a wizard to build your own provider that will build out a custom landing page, and UI components, logos, etc, and you can add the Oauth URLs, client IDs/secrets, so then you can just add any provider, request custom scopes, store the access tokens for API usage from those providers, etc.

**Describe alternatives you've considered**
Others have suggested creating your own cognito pools, making custom lambdas to do the work, etc, but that to me defeats the purpose of using Amplify, to having to do the infrastructure behind the basics that are already provided already.

Start by reviewing the existing authentication-provider support in Amplify Studio and the requested providers listed in the issue, including Twitch, Twitter, GitHub, Apple, Game Center, Microsoft, and Steam. The issue is complete only when the scope is narrowed and either the selected providers are supported or a custom-provider workflow with setup guidance is delivered.

Written by the indexing model from the issue text.