[sync] Validate json blob before posting it to syncAPI

[aaperis]

Describe the bug

In order to avoid errors on the syncAPI like this one:

time="2024-12-12T12:32:46Z" level=info msg="map[error:eror on JSON validation: [I#] [S#] doesn't validate with https://github.com/neicnordic/sensitive-data-archive/tree/master/sda/schemas/bigpicture/file-sync.json#\n  [I#/dataset_files] [S#/properties/dataset_files/type] expected array, but got null\n  [I#/user] [S#/properties/user/minLength] length must be >= 5, but got 0]"

it would be good to add the json validation step also on the sync side.

Steps to reproduce

Expected behavior

• Tests verifying the fix are added

Additional context

Estimation of size

small/medium/big

Estimation of priority

low/medium/high