@Nullable
        @Override
        public Credential apply(String keyName) {
            final CriteriaSet cs = new CriteriaSet();
            cs.add(new EntityIdCriterion(keyName));
            try {
                return resolver.resolveSingle(cs);
            } catch (Throwable cause) {
                return Exceptions.throwUnsafely(cause);
            }
        }
An adapter for {@link CredentialResolver} which helps to resolve a {@link Credential} from
the specified {@code keyName}.
saml/src/main/java/com/linecorp/armeria/server/saml/SamlServiceProviderBuilder.java