[PoC] Add support for DataTrees

[alxthm]

Add support for xr.DataTrees in addition to xr.Dataset, for the xpublish.Rest class.

This change is focused on unlocking support for multiscale overviews in plugins (see #92).

It is a proof of concept intended to support discussion.

Changes:

• Extend xpublish Dependencies with DataTrees. New FastAPI dependencies get_datatree_ids and get_datatree are added and plugins can access them via the hooks (Dependencies.datatree_ids / Dependencies.datatree).
• In the Rest class, add a '/datatrees/{datatree_id}' route prefix, for the above dependency.

This will allow plugins to define routes that serve DataTree data:

class TilesPlugin(Plugin):
    ...

    @hookimpl
    def datatree_router(self, deps: Dependencies):
        """Tiles endpoints for DataTree-based multiscale pyramids."""
        router = APIRouter(
            prefix=self.datatree_router_prefix, tags=self.datatree_router_tags
        )

        @router.get("/{tileMatrixSetId}/{tileMatrix}/{tileRow}/{tileCol}")
        async def get_datatree_tile(
            ...,
            datatree: DataTree = Depends(deps.datatree),
        ):
            ...

[mpiannucci]

Thanks @alxthm ! Myself and @abkfenris will take a deep look at this soon!