### 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boolean property, as well as the `expiration` property that contains the UNIX timestamp past of which the key is no longer valid for authentication.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
return result;
        }
        return userRoleDescriptors;
    }
    /**
     * Builds a comma delimited string from the given string values (e.g. value1, value2...).
     * The number of values included can be controlled with the {@code limit}. The limit must be a positive number.
     * Note: package-private for testing
     */
    static String buildDelimitedStringWithLimit(final int limit, final String... values) {
        if (limit <= 0) {
            throw new IllegalArgumentException("limit must be positive number");
        }
        if (values == null || values.length <= 0) {
            return "";
        }
        final int total = values.length;
        final int omitted = Math.max(0, total - limit);
        final int valuesToAppend = Math.min(limit, total);
        final int capacityForOmittedInfoText = 5; 
        final int capacity = valuesToAppend + (omitted > 0 ? capacityForOmittedInfoText : 0);
        final StringBuilder sb = new StringBuilder(capacity);
        int counter = 0;
        while (counter < valuesToAppend) {
            sb.append(values[counter]);
            counter += 1;
            if (counter < valuesToAppend) {
                sb.append(", ");
            }
        }