[Feature]: Tree: allow async function for get-children

[haexhub]

Describe the feature

Hi,

it would be nice, if we could use an async function to retrieve children items of the tree component.
For example, I want to use this component to show a directory structure of an S3 bucket. To get the child directories I need to do an async http request, which I would like to use as the get-children function. But thats not possible at the moment.
Would be great if it will be some day :smi

Additional information

• I intend to submit a PR for this feature.
• I have already implemented and/or tested this feature.