lava-tool should use XDG directories

With the new work done in lava-tool, right now we are storing user inputs in a file called ".lavaconfig" in the user's home directory:
we should switch to use XDG directories for this kind of configuration files.

Also, we should rethink how this file is called and where to actually store the various parameters asked: not all of them can be considered configurations.

We should probably use the $XDG_CACHE_HOME directory to store parameters that can change from job/testdef, and $XDG_CONFIG_HOME to store those values that will hardly change (lava server URL, RPC endpoint, user name).