Automatically check file integrity using a hash

[SamuelPull]

• The feature was discussed in discussions of this repository.
• I have searched the issues of this repository and believe that this is not a duplicate.

Description 💡

Is someone replaces an uploaded file directly in a blob container or bucket, giving it the same filename and metadata, this is not detected by TruBudget automatically.
Replaced file fails the hash verification initiated manually via the user interface, because hash associated with a document in a workflow item is created on file upload using the api.

• TruBudget api should check integrity of uploaded files automatically by recomputing their hashes and comparing against data stored on chain. This can be done on file download or any file access? Details depend on performance. We have to take into account that files may be large ~70 MB (plus 30% for base 64 encoding).
  setDocumentAvailability looks like a good place
• UI should inform the user when a hash of a file in storage doesn't match the hash stored on chain

I'm not sure about the role of file secret metadata, as this can be easily copied from one blob to a different blob, and this way it will match a file secret recorded on chain. I think this needs re-evaluation? Maybe I misunderstood its use.