Allow field definitions come from other sources than content types in Azure Search AI

[MikeAlhayek]

Is your feature request related to a problem? Please describe.

Currently, the implementation of Azure Search AI does not allow extensibility and require the user to select content type to create index. But, what if we want to create index for that outside content items. Currently there is no option to support this.

Describe the solution you'd like

Change the Index management UI to use display drivers instead of using standard view model approach. Also, do not require a content type. Instead, allow at least one source for the index fields. With this approach, other features can define new source by implementing IContentItemIndexHandler and maybe a new event IAzureAISearchFieldSourceEvents.