core: add new `init_script_path` option: `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.

You'll find install instructions in documentation (see doc/installation.rst).

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