oauth2Client Test Support should allow configuration of principal name

`oauth2Client()` and `mockOAuth2Client()` both allow the configuration of an access token as well as a client registration.

An `OAuth2AuthorizedClient` also requires a principal name in its constructor. By making it configurable, other test support, like OAuth 2.0 Login and OIDC Login, can make sure that the token they register is tied to the user they create.