[Minor][Client Auth] Refactor the client Authenticators to account for the new "ClientAuthenticator" interfaces

[pavankrish123]

Is your feature request related to a problem? Please describe.
As a part of [Removing protocol specific interfaces ] (open-telemetry/opentelemetry-collector#4239) client authenticators will have to implement top level "ClientAuthenticator" which now comprises of all the protocol interfaces.

** Note **

Current modules that implement client authenticator are BearerTokenAuth, Oauth2ClientAuth and since go manages interfaces implicitly there may not be a lot changes that are needed (unless client auth modules are implementing specific protocol only)