Allow the "Custom value" token property to be added to a specific datasource

Problem/Motivation

By default the "custom_value" property can only be added to the General datasource.

However sometimes, the API backend requires the special token to only be used in a specific datasource, e.g. the ai_search backend, where the custom token needs to be added as a "Contextual content", but only for a specific datasource.

Proposed resolution

Allow users to add the custom value to the General tab, as well as into specific entity datasources.

Remaining tasks

Provide MR.