[4.1.x][Feature] Relationship column fluent ->withInlineCreate() method

[tabacitu]

Thanks to the fluent syntax, I think it'll be possible to have a convenience method that adds the InlineCreate operation to the relationship field, which would have the exact same syntax for both single and multiple relationships.

I think this would go a long way towards fixing problems like https://github.com/Laravel-Backpack/CRUD/issues/2597 - I don't think it's easy enough to use InlineCreate right now...