Read client_id from a file in the config folder

[rashil2000]

Related: #537

This PR introduces a default lookup for a file containing the client_id, eliminating the need to add an external command to read it from the filesystem.

It is a plaintext file called client_id present alongside app.toml (in the ~/.config/spotify-player) directory, which contains the ID - if such a file exists we can read it first, and then fallback to other mechanisms.

My main motivation for this change is that shelling out to an external command is an expensive operation, and not all systems have straightforward ways to do that (i.e. Windows does not have the cat command, although there are ways to achieve similar functionality via the findstr or more commands). Of course this is assuming the commands are being invoked without a shell (where stuff like echo is available). But external shell -> external command is even more expensive.