Add the ability to share signature requests

[DazeEnd]

Is your feature request related to a problem? Please describe.
It would be nice if signature requests could be shared with other users who are not signers.

For example, in an office setting there may be two coworkers: Staff1 and Staff2. Staff1 requests the signature of a client ("Client") on a document. It would be nice if there was a way for Staff1 to share the signature request with Staff2 so that Staff2 could see if the document has been signed yet.

Ideally, this sharing mechanism would have a permission set so that the signature request could be shared as either read-only (i.e. see the status of the signature) or full edit permission (which would allow Staff2 to modify the signature request by adding additional signers or repositioning signature boxes.)

This would be great for teams that are working with LibreSign.

Describe the solution you'd like
I think it would be easiest (for the end user) to use the existing sharing features that are built into Nextcloud Files. If a user, "Staff2", has a file shared with him (either the file directly, or a folder containing the file), then Staff2 has permission to any attached signature requests as well. A read-only share would then restrict Staff2 to viewing the LibreSign status. A share that grants "edit" permission to Staff2 would allow Staff2 to modify any attached signature request (signers, signature placement, etc.) People who have received a signature request share should receive email notifications, just as the requester of the signature currently does.

Describe alternatives you've considered
It would also be conceivable that a completely separate permission system be built for LibreSign, but I fear that having two separate permission systems, probably using similar terms, would be confusing for users (especially casual users).

Additional context
N/A