[data grid] Implement server-side data source in tree data

[flaviendelangle]

Part of #8179

This issue addresses the implementation of the server-side tree data in the Data Grid that comes under the umbrella of the data source.

The feature is available in the latest version of the Data Grid.

Check the official documentation here: https://mui.com/x/react-data-grid/server-side-data/tree-data/

[mbiggs-gresham]

I would love to see this feature implemented. We currently are doing an infinite scroll grid in the pro version and have just seen the addition of the tree data and was wondering how we could make the two work together.

Would you envision it would be possible to make it so that not ALL children have to be fetched on the expansion. We are likely to see some very large numbers of child rows (but only 1 level deep), so i'm just wondering if we could somehow make it so that we can retrieve the child rows as part of the infinite scroll.

[flaviendelangle]

The next release will add #3604 which will help lazy loading, I will add an example in the doc of what can be done in your applications without any big changes on our side.

But it will still have limitations.
Infinite loading can't work right now outside of top level rows, so your use-case is probably not doable.
@DanailH this is clearly something to have in mind when we will improve the infinite loading and infinite loading examples.
You might be able to do some "hacky" things to make it work.

[flaviendelangle]

In #4851 we are talking about modifications on the internal state structure to be able to do lazy loading properly.