Make the primary hash of an objstorage configurable

The primary hash (aka the has used as primary key for the objstorage backend) used to be hardcoded as sha1, this makes it configurable among supported hashes (sha1, sha1_git, sha256 and blake2s256).

For the remote api to work, a new /config endpoint has been added allowing a RPC client to ask the server for a config (generic dict), in which the "primary_hash" key is used to configure the client with the same primary hash as the server. If not such endpoint is available, defaults to "sha1".

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