dms: Allow uploading to shared folder

[diggy128]

Is your feature request related to a problem?
Sharing a folder allows viewing the already existing files.
There is no way to allow uploading to that folder.

Describe the solution you'd like
Allow non odoo user (visitor that uses the tokenized link) to upload files to the directory.
There should also be a quota for the directory (per token or directory)

Describe alternatives you've considered
Share a cloud url (e.g. Google drive) -> create storage for that target (in fs_storage) -> use that storage in dms (dms_storage).
This however adds dependency on external services and adds (sometimes unneeded) complexity.

Additional context
There are many cases where exchange of files is necessary between Odoo users and external entities (either portal users or non Odoo users).
Consider a typical draft / review / approval procedure for any file that can't fit in an email (e.g. a packaging design or a technical drawing) using the purchase order object in Odoo:

1. External partner (partner A) prepares the document
2. File is sent to our company (company) with a file transfer service (e.g. wetransfer) or a link to a cloud service (most likely link to file), most likely with an email mentioning the link
3. We open the email and download the file from the link
4. We attach the email to the purchase order
5. We attach the downloaded file to the purchase order (at this point the link between the email and the file is lost)
6. We make changes to the file and re-attach to purchase order
7. We create a share link for the file
8. We send a message to partner A from the purchase order sending the shared file link.
9. Partner A reviews
10. We start again from 2. until the file is approved.

So even though the files and emails are there, there is no clear connection and file context is not clear.
Allowing to share a writable directory to an external / non user would allow to keep all file exchange in Odoo without any external service.
Finally, consider privacy complications when external partners use their preferred cloud service, and even data retention policies.

[pedrobaeza]

The main challenge here is avoid abuses, so at least a maximum size should be set.