verifyWithOAuth2

suspend fun verifyWithOAuth2(
    credential: UserPasswordCredential, 
    client: HttpClient, 
    settings: OAuth2ServerSettings
): OAuth2

Implements Resource Owner Password Credentials Grant see http://tools.ietf.org/html/rfc6749#section-4.3

Takes UserPasswordCredential and validates it using OAuth2 sequence, provides OAuthAccessTokenResponse.OAuth2 if succeeds