[C++][FlightRPC] Optionally cache serialized ListFlights serverside

ListFlights serializes flights each time it is called. If we have many flights and ListFlights is called often this can produce a significant load on the server. We could have an optional cache server side to avoid this.

**Reporter**: [Rok Mihevc](https://issues.apache.org/jira/browse/ARROW-15764) / @rok
**Watchers**: [Rok Mihevc](https://issues.apache.org/jira/browse/ARROW-15764) / @rok

_{**Note**: *This issue was originally created as [ARROW-15764](https://issues.apache.org/jira/browse/ARROW-15764). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by locating the FlightRPC ListFlights entry point and tracing where flights are serialized on each call. Define the optional server-side cache behavior and its invalidation or refresh expectations before implementing it. Done means repeated ListFlights calls can avoid repeated serialization when caching is enabled, without changing the uncached behavior.

Written by the indexing model from the issue text.