feat: assistant role (#1588)

## Description

Introduces the ability for users with the **Assistant** role to prefill
fields on behalf of other signers. Assistants can fill in various field
types such as text, checkboxes, dates, and more, streamlining the
document preparation process before it reaches the final signers.

https://github.com/user-attachments/assets/c1321578-47ec-405b-a70a-7d9578385895

packages/prisma/schema.prisma

@@ -417,6 +417,7 @@ enum RecipientRole {
   SIGNER
   VIEWER
   APPROVER
+  ASSISTANT
 }
 
 /// @zod.import(["import { ZRecipientAuthOptionsSchema } from '@documenso/lib/types/document-auth';"])