RFC for SAML Integration

**Is your feature request related to a problem? Please describe.**
Yes, we have several clients that need SAML integration with their Amplify application and without the support, we had to create a custom solution.

**Describe the solution you'd like**
An option in the auth plugin to allow you to set up SAML authentication just like you can with OIDC, username/password, etc.

**Describe alternatives you've considered**
We created a custom solution that requires some manual work in the AWS console and hacky updates to the auto-generated CloudFormation templates that are cumbersome to work with.

With more time, we've considered creating a custom plugin under AWS labs that would allow this, though it seems like a better fit for the auth CLI if it's a common request or need.

**Additional context**
My teammate @brianhubbell at ProServe would like to spec and build this out and just need some guidance on how you think we should approach this - plugin vs adding to auth - and the process for spec-ing the API before building it, etc.

No implementation file or test is named. Start by reviewing the auth plugin and auth CLI's existing OIDC flow, then compare the plugin and CLI approaches described in the issue. Done would be an agreed direction and API specification for SAML support.

Written by the indexing model from the issue text.