[Feature] expose a keyword for manual cache key

[liamhuber]

In combination with cache_directory, it would be nice if there was an option to manually provide the key at which the cache result would be available, as a non-default alternative to automatically generating it by hashing the cloudpickle binary.

E.g. the under-the-hood change would probably look very much like

def serialize_funct_h5(
    fn: Callable,
    fn_args: Optional[list] = None,
    fn_kwargs: Optional[dict] = None,
    resource_dict: Optional[dict] = None,
    key: Optional[str] = None,
) -> tuple[str, dict]:
    """
    Serialize a function and its arguments and keyword arguments into an HDF5 file.

    Args:
        fn (Callable): The function to be serialized.
        fn_args (list): The arguments of the function.
        fn_kwargs (dict): The keyword arguments of the function.
        resource_dict (dict): resource dictionary, which defines the resources used for the execution of the function.
                              Example resource dictionary: {
                                  cores: 1,
                                  threads_per_core: 1,
                                  gpus_per_worker: 0,
                                  oversubscribe: False,
                                  cwd: None,
                                  executor: None,
                                  hostname_localhost: False,
                              }

    Returns:
        Tuple[str, dict]: A tuple containing the task key and the serialized data.

    """
    if fn_args is None:
        fn_args = []
    if fn_kwargs is None:
        fn_kwargs = {}
    if resource_dict is None:
        resource_dict = {}
    if key is None:
        binary_all = cloudpickle.dumps(
            {"fn": fn, "args": fn_args, "kwargs": fn_kwargs, "resource_dict": resource_dict}
        )
        task_key = fn.__name__ + _get_hash(binary=binary_all)
    else:
        task_key = key
    data = {
        "fn": fn,
        "args": fn_args,
        "kwargs": fn_kwargs,
        "resource_dict": resource_dict,
    }
    return task_key, data