Some operations return str, but there are proper response models

[csadorf]

I noticed that we are returning str on a few operations, e.g., here:

python-sdk/marketplace/app/v0/object_storage.py

Lines 72 to 82 in 5923469

	@check_capability_availability
	def create_collection(
	self,
	collection_name: object_storage.CollectionName = None,
	metadata: dict = None,
	) -> str:
	return self._client.put(
	self._proxy_path("createCollection"),
	params={"collection_name": collection_name} if collection_name else {},
	headers=_encode_metadata(metadata),
	).text

However, according to the API, the response is actually also a JSON object. Can we fix that inconsistency, please?

Originally posted by @csadorf in #43 (review)