Mostly addresses #15027

dask/dask-expr#728 exposed the necessary mechanisms for us to define a custom dask-expr backend for `cudf`. The new dispatching mechanisms are effectively the same as those in `dask.dataframe`. The only difference is that we are now registering/implementing "expression-based" collections.

This PR does the following:
- Defines a basic `DataFrameBackendEntrypoint` class for collection creation, and registers new collections using `get_collection_type`.
- Refactors the `dask_cudf` import structure to properly support the `"dataframe.query-planning"` configuration.
- Modifies CI to test dask-expr support for some of the `dask_cudf` tests. This coverage can be expanded in follow-up work.

~**Experimental Change**: This PR patches `dask_expr._expr.Expr.__new__` to enable type-based dispatching. This effectively allows us to surgically replace problematic `Expr` subclasses that do not work for cudf-backed data. For example, this PR replaces the upstream `TakeLast` expression to avoid using `squeeze` (since this method is not supported by cudf). This particular fix can be moved upstream relatively easily. However, having this kind of "patching" mechanism may be valuable for more complicated pandas/cudf discrepancies.~

## Usage example

```python
from dask import config
config.set({"dataframe.query-planning": True})
import dask_cudf

df = dask_cudf.DataFrame.from_dict(
    {"x": range(100), "y":  [1, 2, 3, 4] * 25, "z": ["1", "2"] * 50},
    npartitions=10,
)
df["y2"] = df["x"] + df["y"]
agg = df.groupby("y").agg({"y2": "mean"})["y2"]
agg.simplify().pprint()
```
Dask cuDF should now be using dask-expr for "query planning":
```
Projection: columns='y2'
  GroupbyAggregation: arg={'y2': 'mean'} observed=True split_out=1'y'
    Assign: y2=
      Projection: columns=['y']
        FromPandas: frame='<dataframe>' npartitions=10 columns=['x', 'y']
      Add:
        Projection: columns='x'
          FromPandas: frame='<dataframe>' npartitions=10 columns=['x', 'y']
        Projection: columns='y'
          FromPandas: frame='<dataframe>' npartitions=10 columns=['x', 'y']
```

## TODO

- [x] Add basic tests
- [x] Confirm that general design makes sense

**Follow Up Work**:

- Expand dask-expr test coverage
- Fix local and upstream bugs
- Add documentation once "critical mass" is reached

Authors:
  - Richard (Rick) Zamora (https://github.com/rjzamora)
  - Lawrence Mitchell (https://github.com/wence-)
  - Vyas Ramasubramani (https://github.com/vyasr)
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - Lawrence Mitchell (https://github.com/wence-)
  - Ray Douglass (https://github.com/raydouglass)

URL: #14805