### Description

The [Get API key information API](https://www.elastic.co/guide/en/elasticsearch/reference/master/security-api-get-api-key.html) can currently filter the returned keys by:
 * key id/name
 * keys owned by the calling user
 * keys owned by other users, by `realm_name` and `username`

Irrespective of the filtering used, both invalidated and expired (but not yet deleted) keys will be included in the response.
In the response, keys have the `invalidated` boolean property, as well as the `expiration` property that contains the UNIX timestamp past of which the key is no longer valid for authentication.

I propose we introduce a way for the API to support retrieving:
* only valid API keys (not return invalidated keys) - using a boolean request parameter
* keys not expired before a certain timestamp - using a timestamp parameter that defaults to "now" - also note that keys' experiation date is always checked against the local node's clock
x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/security/authc/ApiKeyServiceTests.java
+ ". Remote indices are not supported by all nodes in the cluster. "
                    + "Use the update API Key API to re-assign remote indices to the API key(s), after the cluster upgrade is complete."
            );
        }
    }
    public void testMaybeRemoveRemoteIndicesPrivilegesWithSupportedVersion() {
        final String apiKeyId = randomAlphaOfLengthBetween(5, 8);
        final Set<RoleDescriptor> userRoleDescriptors = Set.copyOf(
            randomList(1, 3, ApiKeyServiceTests::randomRoleDescriptorWithRemoteIndexPrivileges)
        );
        
        final TransportVersion minTransportVersion = TransportVersionUtils.randomVersionBetween(
            random(),
            TRANSPORT_VERSION_ADVANCED_REMOTE_CLUSTER_SECURITY_CCS,
            TransportVersion.current()
        );
        final Set<RoleDescriptor> result = ApiKeyService.maybeRemoveRemoteIndicesPrivileges(
            userRoleDescriptors,
            minTransportVersion,
            apiKeyId
        );
        
        assertThat(result, equalTo(userRoleDescriptors));
    }
    public void testBuildDelimitedStringWithLimit() {
        int limit = 2;
        assertThat(ApiKeyService.buildDelimitedStringWithLimit(limit), equalTo(""));
        assertThat(ApiKeyService.buildDelimitedStringWithLimit(limit, new String[] {}), equalTo(""));
        assertThat(ApiKeyService.buildDelimitedStringWithLimit(limit, "id-1"), equalTo("id-1"));
        assertThat(ApiKeyService.buildDelimitedStringWithLimit(limit, "id-1", "id-2"), equalTo("id-1, id-2"));