Move NativeAuth.start() into the NativeAuth protocol declaration

[ejensen]

Changes

Moves NativeAuth.start() into the NativeAuth protocol declaration. This allows SDK clients to provide an implementation. Rather than restricting it to the default implementation defined in the protocol extension.

This enables clients to write their own implementation of start. For example, we need to call authentication.login(facebookSessionAccessToken: ...) in our implementation of the protocol, rather than authentication.loginSocial(token:...) that's called by the default protocol extension implementation.

This is a non-breaking change to the protocol.

References

• Methods defined in a protocol and provided a default implementation are dynamically dispatched (can be overridden)
• Methods only defined in a protocol extension are always statically dispatched (can't be overridden)
• Swift Evolution Board discussion on static vs dynamic dispatch of protocol extensions

Testing

No additional testing is required since this is an inert change to a protocol

• 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

[Widcket]

@ejensen thanks for the contribution!