Improve OIDC compliance [SDK-987]

[Widcket]

Changes

This update improves the SDK support for OpenID Connect. In particular, it modifies the sign in verification phase by substituting backchannel based checks with id_token validation.

This PR aggregates a series of previously reviewed and approved PRs:

• Added IDTV documentation to the README [SDK-1670] #224
• Improve OIDC compliance: add claims validation support [SDK-987] #223
• Improve OIDC compliance: add signature validation support [SDK-987] #222

Public surface area

Additions

• A new public method in Auth0::Client called validate_id_token that performs ID Token validation.
• Auth0::Algorithm::HS256 and Auth0::Algorithm::RS256, classes that represent the signing algorithms and encapsulate the logic to obtain the respective secret/public keys.
• A new exception Auth0::InvalidIdToken.

Changes

None.

Testing

• This change adds unit test coverage
• This change adds integration test coverage
• This change has been tested on the latest version of Ruby

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors
• Rubocop passes on all added/modified files
• All active GitHub checks have passed