callbacks: replace kwargs positional argument with arbitrary kwargs in branched

[skshetry]

The following would be an example of someone using a branched callback.

with callback.branched(path_1, path_2, {}) as child:
    pass

The kwargs feel unnecessary to me. So I am proposing to remove that. It can be added later if someone requests it.

Also see #1460 (comment).

[martindurant]

Since it's brand new, I suppose it's OK, but I mildly thing =None or **kwrags is better.

After all this, we should probably update text at https://filesystem-spec.readthedocs.io/en/latest/features.html#callbacks for users and somewhere better describe the process to write new callbacks.

[skshetry]

Since branched() will be used deep inside put/get, I can't think of a good way to pass kwargs to it. Neither can I think of a good reason to do so. So far, the only use case in branch was to add a child callback.

[skshetry]

But on isolation, **kwargs may be more ergonomic and extensible.

[skshetry]

Before, this could be used to influence kwargs passed to put_file/get_file. I am not sure what the use case for that was, except of adding callback kwarg.

So this is deviating with branch in that regard.

[martindurant]

So you vote to remove and add back later if there is a use case?

[skshetry]

I don't see a use case for mutating kwargs in branched(). So, I'd just go with **kwargs (i.e. this PR).

I don't have any strong opinion against kwargs=None as it is similar but less ergonomic. But it does provide a way of mutating kwargs (but I don't see a use case for it).

We could also drop the third argument entirely, but that is not very flexible for future changes (I know I started with that, but 🤷🏽).