[v16] Move Server Scripts to the file system about frappe HOT 1 OPEN

Slightly modified proposal:

• Leave old doctypes as is, introduce new doctype Extension Scripts or just Extensions (naming can be dealt with later)
• Script types based on folder structure

- /scripts
    - /doctype_events
        - Sales Order.py (all events moved inside single file like controllers)
    - /api
        - path.py
    - /cron
        - crontab (cron -> file.py) 
        - script.py

Execution:

• Use exec directly instead of safe_exec i.e. allow everything.
• Not allowed in shared environment (same as current server script v15)
• Provide a flag for using old syntax OR write a code-rewriter for old script to migrate to normal python syntax instead of safe_exec ones (quite hard IMO)
• Once this is ready and we are confident about it, migrate server script contents to file and disable old doctype.

Pros:

• better security model
• version controllable
• full access to vanilla python, libraries, imports

Cons:

• Without a UI editor this can be quite a pain for users to edit using SSH. This can be solved by FC. For Frappe docker we can provide VS Code container with some control over who can access it. ( @revant 👀 )
• Allowing full python also means allowing monkey patching using this, which eventually bigbraindevs will do. 🌚
• breaking & migration woes

from frappe.