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