`get_handler` can now return several handlers, which is notably useful for component with
several included services.
Various improvements such move to `async` method, docstring/type hints update, and slight
improvment of SatXMPPComponent.
rel 453
this directory may be used to share files between backend and frontends. Normally, an
os-dependent temporary directory is created for that, but if this option is set, the
directory will be created in <local_dir>/<cache_dir>, which may be useful in some use case
(e.g. containerized frontends and backend).
`init_script_path` option can be used in `[DEFAULTS]` to run a script at the end of
backend initialisation. A new `init_pre_script` method is used to wait for backend to
reach this stage (designed to be used mostly by CLI frontend), then the usual `ready_get`
method is finished once the script is finished.
