Support passing extra arguments to the paccache service from a "config" file

The only way to pass extra arguments to the paccache servers currently is by overriding ExecStart= with a drop-in file.

It may make sense to include a env file (with EnvironmentFile=-), which can then be easily modified for passing extra arguments to paccache.

This approach is already using by existing packages.

mentioned in issue archlinux/infrastructure#649 (closed)

added enhancement label

mentioned in commit antiz/pacman-contrib@9fc1b142

mentioned in commit antiz/pacman-contrib@933e60e6

mentioned in merge request !53 (merged)

I opened !53 (merged) to address this :)

mentioned in commit antiz/pacman-contrib@39f595f2

mentioned in commit antiz/pacman-contrib@60f2bdfb

mentioned in commit 7db89391

closed with merge request !53 (merged)