Proposed credential verification API

RealmIdentity.java

public interface RealmIdentity {
//  [...]
    /**
     * Verify the given credential.  The result is one of the following:
     * <ul>
     *     <li>{@link VerificationResult#DENIED} - the credential is not valid</li>
     *     <li>{@link VerificationResult#UNVERIFIED} - the credential is not supported or could not be verified; retry with another credential type</li>
     *     <li>{@link VerificationResult#VERIFIED} - the credential is valid and verified</li>
     * </ul>
     *
     * @param credential the credential to verify
     * @return the non-{@code null} verification result
     */
    VerificationResult verifyCredential(Object credential);
//  [...]
}