feat: configurable allow-list for dbus interfaces

[hojjatabdollahi]

Based on our conversation here.

This allows adding a toml file to /usr/share/cosmic-comp/ei-clients.d/ and whitelist more bus names. It leaves the hardcoded ones alone.

The config is only read on startup, so after installing a new package, you have to log out and in for the app to have access.

For example my remote desktop app installs /usr/share/cosmic-comp/ei-clients.d/20-cxrd.toml with this content:

[[ei.client]]
bus_name = "io.github.hojjatabdollahi.cxrd"
device_types = ["keyboard", "pointer", "touchscreen"]

I also added a11y_keyboad_monitor as another example of how to make hardcoded allowlists configurable.

I didn't use cosmic-conf because I wanted user writable directories to be excluded.
And I didn't work on setting up a watcher and reloading in real time. It only loads on startup.

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