Added support for custom issuer for ID Token validation [SDK-1909]

[Widcket]

Changes

New feature

This PR adds support for specifying a custom issuer for ID Token validation.

Auth0.webAuth()
    .issuer("https://example.com/")
    .start { result in
    // Handle result
}

Bugs fixed

In the course of testing this new feature, a couple of issues arised regarding the implementation of ID Token validation:

• The validation was not being executed for ID Tokens received with the code exchange when there was no previous front-channel ID Token (case of responseType = [.code], which is the default value).
• The signature validation was failing due to the DER encoding logic not adding a leading 0 to the RSA modulus. This is required to indicate that the number is not negative. Additionally, the test JWK generator logic was not removing the leading 0 when decoding a DER-encoded RSA public key.

Other changes

• Linter warnings regarding the order of the getters and setters were fixed on _ObjectiveWebAuth.swift.
• A compiler warning regarding a dangling pointer was fixed on ChallengeGeneratorSpec.swift.

Testing

• 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