Query API Keys by application

In support of https://github.com/elastic/kibana/issues/77966, we are seeking to list API Keys for use with APM. Currently API keys can be retrieved by a few attributes - [id, name, realm, user, owner](https://www.elastic.co/guide/en/elasticsearch/reference/current/security-api-get-api-key.html#security-api-get-api-key-path-params) - is it feasible to add application?

All keys used by APM for server - agent authorization are in the `"apm"` application.

Start with the linked Get API Key documentation and the supporting Kibana issue, then trace the Elasticsearch security API path that retrieves API keys. Determine how an application filter would fit the existing id, name, realm, user, and owner attributes; done means API keys used by the `apm` application can be listed reliably and the behavior is documented.

Written by the indexing model from the issue text.