Let generic authenticator calculate cache TTL based on the response from the identity_info_endpoint

[dadrus]

As of today the cache TTL (if configured) is static. That might be an issue if the response provided by the identity_info_endpoint is valid in a specific time frame only and the configured cache TTL makes the authenticator think, the cached response if valid, even it is not any more.

Since different system (behind the identity_info_endpoint) have different fields and formats for validity information, the solution must support not only the extraction of the relevant values from the response, but also support different formats.