### 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boolean property, as well as the `expiration` property that contains the UNIX timestamp past of which the key is no longer valid for authentication.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/main/java/org/elasticsearch/xpack/security/action/apikey/TransportGetApiKeyAction.java
TransportService transportService,
        ActionFilters actionFilters,
        ApiKeyService apiKeyService,
        SecurityContext context
    ) {
        super(GetApiKeyAction.NAME, transportService, actionFilters, GetApiKeyRequest::new);
        this.apiKeyService = apiKeyService;
        this.securityContext = context;
    }
    @Override
    protected void doExecute(Task task, GetApiKeyRequest request, ActionListener<GetApiKeyResponse> listener) {
        String[] apiKeyIds = Strings.hasText(request.getApiKeyId()) ? new String[] { request.getApiKeyId() } : null;
        String apiKeyName = request.getApiKeyName();
        String username = request.getUserName();
        String[] realms = Strings.hasText(request.getRealmName()) ? new String[] { request.getRealmName() } : null;
        final Authentication authentication = securityContext.getAuthentication();
        if (authentication == null) {
            listener.onFailure(new IllegalStateException("authentication is required"));
        }
        if (request.ownedByAuthenticatedUser()) {
            assert username == null;
            assert realms == null;
            
            username = authentication.getEffectiveSubject().getUser().principal();
            realms = ApiKeyService.getOwnersRealmNames(authentication);
        }
        apiKeyService.getApiKeys(realms, username, apiKeyName, apiKeyIds, request.withLimitedBy(), listener);
    }
}