Make on-demand generating of embedding an opt-in option

Problem/Motivation

It could be that despite caching, some sites would never want embedding generation. At the moment its set as a fallback. Add the ability to opt out completely, and set this as the default for new sites.

Steps to reproduce

Look at AI Related Node Filter

Proposed resolution

Add checkbox to opt-in

Remaining tasks

MR

User interface changes

New option to opt in to generating embedding on demand when a source vector database is chosen

API changes

N/A

Data model changes

N/A