WIP: ✨ Add cred dropdown for custom OAuth scope levels

[pemontto]

Briefly discussed in discord - https://discord.com/channels/832547762716278804/832547763191152642/1026463938692517888

Add OAuth2 scope options so users can limit their exposure. Generally useful to limit the blast radius of n8n creds, and especially important when credential sharing becomes a thing.

In this example with Google Drive a user can choose:

1. Read & Write: same as the original default. Access to create, read, modify, and delete all files
2. Read Only: ability to read and list all available files
3. n8n Files Only: create, read, modify, and delete all files created by n8n only

[pemontto]

I even would take a toggle like Customise Scopes, or Advanced Settings that would unhide the scopes field 🙏