Migrate all errors to structs [SDK-2968]

[Widcket]

Changes

⚠️ THIS PR CONTAINS BREAKING CHANGES

In Auth0.swift 1.x WebAuthError and CredentialsManagerError are enums, and as such the consumers must switch exhaustively over all the cases. This means that adding new cases constitutes a breaking change. So this was the source of many TODO: comments, as new error cases needed to be added.

This PR migrates all errors to be structs, and in the case of WebAuthError and CredentialsManagerError in particular, all the former cases are now static properties, meaning that they can be switched over but that switch can never be exhaustive. So adding new error cases will not be a breaking change.

Additionally, the error handling DX was improved, as now all errors conform to Equatable and CustomDebugStringConvertible, and all errors are being properly wrapped. Before, Web Auth could produce either a WebAuthError or an AuthenticationError; now it will only produce a WebAuthError. Same for the Credentials Manager, which will only produce a CredentialsManagerError.

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