### 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boolean property, as well as the `expiration` property that contains the UNIX timestamp past of which the key is no longer valid for authentication.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
{
              "doc_type": "api_key",
              "creation_time": 1591919944598,
              "expiration_time": null,
              "api_key_invalidated": false,
              "api_key_hash": "{PBKDF2}10000$abc",
              "role_descriptors": {},
              "limited_by_role_descriptors": {
                "limited_by": {
                  "cluster": [ "all" ]
                }
              },
              "name": null,
              "version": 7000099,
              "creator": {
                "principal": "admin",
                "metadata": {},
                "realm": "file1"
              }
            }""";
        final ApiKeyDoc apiKeyDoc = ApiKeyDoc.fromXContent(
            XContentHelper.createParser(
                NamedXContentRegistry.EMPTY,
                LoggingDeprecationHandler.INSTANCE,
                new BytesArray(apiKeyDocumentSource),
                XContentType.JSON
            )
        );
        assertEquals(-1L, apiKeyDoc.expirationTime);
        assertNull(apiKeyDoc.name);