Add new option `accounts.contact.accounts.<name>.khard.addressbooks`.
Remove the previous soln,
`accounts.contact.accounts.<name>.khard.defaultCollection`, which is
superseded with the new option.
Add a new test to check the new `addressbooks` option. Modify an
existing test which was checking the removed `defaultCollection`.
Previous commit a38f88 allowed a hardcoded path to be set for khard if
the path set for its local storage is not the actual `vdir`. This was
accomplished via adding the `defaultCollection` option. However this
accepted only a single sub-directory, and when one has more than a
single collection on the same dir this would require repetition on
configuration to set [1].
This is a continuation of the soln given to
nix-community/home-manager#4531, refer to there and the previous PR [2]
for reference.
[1]: https://github.com/nix-community/home-manager/issues/4531#issuecomment-2701156246
[2]: https://github.com/nix-community/home-manager/pull/5220
