Support custom job manager classes in initialize_from_df

[soxofaan]

@mbuchhorn asked for feature to properly support custom job manager classes in initialize_from_df. Which is currently hard-linked to MultiBackendJobManager

openeo-python-client/openeo/extra/job_management.py

Lines 799 to 808 in 37ba260

	# TODO: option to provide custom MultiBackendJobManager subclass with custom normalize?
	if self.exists():
	if on_exists == "skip":
	return self
	elif on_exists == "error":
	raise FileExistsError(f"Job database {self!r} already exists.")
	else:
	# TODO handle other on_exists modes: e.g. overwrite, merge, ...
	raise ValueError(f"Invalid on_exists={on_exists!r}")
	df = MultiBackendJobManager._normalize_df(df)