[Feature Request] VTreeTable component or aggregation support

Problem to solve

There are a lot of applications for a data table which has hierarchical groupings but still retains the tabular columns. It is a blend of v-tree-view and v-data-table, to become a v-tree-table.

I am posting this as a separate feature request, but it is somewhat related to issue #3180. However, I believe it is different enough, and perhaps even encapsulates the functionality that a grouped data table provides. With a v-tree-table, the grouped table may not be needed(??)

Proposed solution

Look at https://docs.webix.com/desktop__treetable.html#apireference as an example of what functionality it provides as well as an API. It is certainly not "vuetified" but it might give some good ideas as to how to implement.

Start by reviewing the related issue #3180 and the Webix treetable API reference linked in the proposal. Define the intended VTreeTable or aggregation scope, API, and acceptance criteria before identifying the relevant Vuetify component entry points; done means the agreed hierarchical table behavior is implemented and documented.