Add `ProtomapsFileHandlerFromFS` method

[straup]

Add a method to create a Protomaps net/http FileHandler from an fs.FS instance.

Ultimately this should take over most of code from ProtomapsFileHandlerFromPath which would then call the new function.

This is the first step to enable loading self-owned PMTiles databases from remote servers (or, more importantly, anything that implements fs.FS).