Improve OIDC compliance: integrate IDTV

[Widcket]

Changes

This update improves the SDK support for OpenID Connect (OIDC). In particular, it hooks the ID Token validation logic in the authentication flows.

What’s being added in this PR

• Tests for the jwks endpoint.

What’s being changed in this PR

• Removed the decodeJwt(_:) method from CredentialsManager.swift and replaced it with calls to JWTDecode logic.
• Changed the signature of the (internal) method validate(idToken:context:signatureValidator:claimsValidator:callback:) to use for as the external name for the parameter context, for the sake of API expressiveness.
• Changed the signature of the initialisers of the (internal) structs ImplicitGrant and PKCE in order to pass the parameters needed to perform ID Token validation.
• Updated existing tests to account for ID Token validation.

Public surface area

• Additions: the methods leeway(_:) and maxAge(_:) in the WebAuth builder.
• Changes: none.

Testing

Please describe how this can be tested by reviewers. Be specific about anything not tested and reasons why. If this library has unit and/or integration testing, tests should be added for new functionality and existing tests should complete without errors.

• This change adds unit test coverage
• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines
• All existing and new tests complete without errors
• All active GitHub checks have passed

[joshcanhelp]

Mostly questions but some might lead to changes.

Nice work on this! Hopefully this was helpful, Swift is difficult for me to read, lots of unfamiliar syntax!

[Widcket]

@joshcanhelp can you please also check the domain logic in this entire file?
It contains the implementations of OAuth2 grants (implicit and code flow).
https://github.com/auth0/Auth0.swift/blob/ca28451be12118c8928745aead45feaa63e669e1/Auth0/OAuth2Grant.swift