[auth]: support oauth client_secret_basic / none / custom methods (#720)

* Allow OAuthClientProvider to control authentication to token endpoint w/ addClientAuthentication (exchange / refresh)

* Include mockProvider in exchangeAuthorization tests.

* feature. Add client_secret_basic and none authentication methods

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Co-authored-by: Jared Hanson <jaredhanson@gmail.com>
Co-authored-by: SightStudio <sdm3285@gmail.com>
Co-authored-by: Claude <noreply@anthropic.com>

Author: 4 people