Add caching configuration

It's not entirely clear for me as an user how the cache works. I keep seeing data usage even for places that should be completely cached by now. How long is location data cached for?

I propose that the caching duration be configurable. This way someone can
a) Figure out how long the cache duration is
b) Modify it to balance privacy and up-to-date-ness

:)

(btw, it is impressive how much dedication and polish is flowing into microg. Thank you for making this!)

No file or test is named. Start by locating the cache implementation and its current location-data lifetime in the AppleWifiNlpBackend repository, then identify how runtime settings are defined. Done means the caching duration can be configured and the effective duration is clear to users.

Written by the indexing model from the issue text.