[Data Storage] Allow downloading objects to public buckets that do not require signed URLs

The current system may enforce the use of signed URLs for object uploads that are stored in private buckets. This issue proposes enabling the upload of data to public or private buckets. The goal of enabling public buckets is to facilitate that the data stored in them -which will basically be data offerings- can be accessed by external parties upon need (e.g., from AIoD, by an EDC connector, from EOSC, etc.)

To support this, we propose adding an optional visibility parameter to the current upload endpoint.

Expected Behavior:

• If visibility is not provided or set to "private", the object is stored in the private bucket as usual.
• If visibility is set to "public", the object is stored in the public bucket and becomes accessible without a signed URL. Listing the files in the bucket still requires credentials.

/cc @jordiarjona, @kyzpanos17 , @kyzpanos