Begin transition from libpurecool to libdyson

Recently Dyson changed their API which broke libpurecool[1]'s integration.
This resulted in prometheus-dyson being unable to enumerate devices via Dyson,
and thus fail to restart successfully.

libdyson refactors libpurecool with a clearer separation between the online
Dyson API & the device-interaction logic. This allows us to perform a one-time
login to Dyson and cache device information locally, removing the need for
repeated logins to Dyson. libdyson also has a more consistent API between
different models.

This change starts the transition by introducing login component (account.py)
and an adapter (libpurecool_adapter) to use the cached information with
libpurecool. This also adds a flag (--create_device_cache) to perform the
login&OTP dance with Dyson and generate the needed configuration.

[1] https://github.com/etheralm/libpurecool/issues/37