### 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boolean property, as well as the `expiration` property that contains the UNIX timestamp past of which the key is no longer valid for authentication.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/authc/ApiKeyService.java
this.inactiveApiKeysRemover = new InactiveApiKeysRemover(settings, client, clusterService);
        this.threadPool = threadPool;
        this.cacheHasher = Hasher.resolve(CACHE_HASH_ALGO_SETTING.get(settings));
        final TimeValue ttl = CACHE_TTL_SETTING.get(settings);
        final int maximumWeight = CACHE_MAX_KEYS_SETTING.get(settings);
        if (ttl.getNanos() > 0) {
            this.apiKeyAuthCache = CacheBuilder.<String, ListenableFuture<CachedApiKeyHashResult>>builder()
                .setExpireAfterAccess(ttl)
                .setMaximumWeight(maximumWeight)
                .removalListener(getAuthCacheRemovalListener(maximumWeight))
                .build();
            final TimeValue doc_ttl = DOC_CACHE_TTL_SETTING.get(settings);
            this.apiKeyDocCache = doc_ttl.getNanos() == 0 ? null : new ApiKeyDocCache(doc_ttl, maximumWeight);
            cacheInvalidatorRegistry.registerCacheInvalidator("api_key", new CacheInvalidatorRegistry.CacheInvalidator() {
                @Override
                public void invalidate(Collection<String> keys) {
                    if (apiKeyDocCache != null) {
                        apiKeyDocCache.invalidate(keys);
                    }
                    keys.forEach(apiKeyAuthCache::invalidate);
                }
                @Override
                public void invalidateAll() {
                    if (apiKeyDocCache != null) {
                        apiKeyDocCache.invalidateAll();
                    }
                    apiKeyAuthCache.invalidateAll();
                }
            });
            cacheInvalidatorRegistry.registerCacheInvalidator("api_key_doc", new CacheInvalidatorRegistry.CacheInvalidator() {