### 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boolean property, as well as the `expiration` property that contains the UNIX timestamp past of which the key is no longer valid for authentication.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/rest/action/apikey/RestGetApiKeyAction.java
final String userName = request.param("username");
        final String realmName = request.param("realm_name");
        final boolean myApiKeysOnly = request.paramAsBoolean("owner", false);
        final boolean withLimitedBy = request.paramAsBoolean("with_limited_by", false);
        final GetApiKeyRequest getApiKeyRequest = GetApiKeyRequest.builder()
            .realmName(realmName)
            .userName(userName)
            .apiKeyId(apiKeyId)
            .apiKeyName(apiKeyName)
            .ownedByAuthenticatedUser(myApiKeysOnly)
            .withLimitedBy(withLimitedBy)
            .build();
        return channel -> client.execute(GetApiKeyAction.INSTANCE, getApiKeyRequest, new RestBuilderListener<>(channel) {
            @Override
            public RestResponse buildResponse(GetApiKeyResponse getApiKeyResponse, XContentBuilder builder) throws Exception {
                getApiKeyResponse.toXContent(builder, channel.request());
                
                if (Strings.hasText(apiKeyId) && getApiKeyResponse.getApiKeyInfos().length == 0) {
                    return new RestResponse(RestStatus.NOT_FOUND, builder);
                }
                return new RestResponse(RestStatus.OK, builder);
            }
        });
    }
    @Override
    public String getName() {
        return "xpack_security_get_api_key";
    }
}