In this commit, we added the envelope feature on `sign.template` by allowing handling multiple attachments in the same template. The changes description and coding choices explanations are described below. 

**SignDocument addition**
For achieving this, we created a new class called `sign.document` and made `sign.template` point to multiple of them. This class stores an attachment and holds the order information in the template, as well points to its `sign.item` records, previously done in `sign.template`. All the rendering code got moved from `sign.template` to `sign.document` since it now stores the attachment, making the `sign.template` code a lot cleaner and easying the adaptation of the JavaScript rendering as well.


**SignCompletedDocument addition**
Before this PR, the storage for the completed document in the `sign.request` was just a single Binary field, and its functions were defined at `sign.template`. In this PR, we added a new model `sign.completed.document` for holding it as moving the document geration over there. This made us keep the simplicity of working with the Binary field (instead of `ir.attachment` directly), we were able to move a function over there making it more maintainable and allowing future improvements such as moving the certificates logic.


task-4555838

Co-authored-by: Gabriel Felix <gdpf@odoo.com>
Co-authored-by: Ahmad Elmaghraby <alah@odoo.com>
Co-authored by: Abdelrahman Fawzy <abdh@odoo.com>