[POC]: Added "pc" fsspec filesystem

[TomAugspurger]

This adds a "pc" fsspec filesystem implementation, which lets us
insert "pc::" in an fsspec URL and automatically sign it when loading
it with an fsspec client.

The primary motivation is integration with fsspec's filesystem where
users would need to call planetary_computer.sign in multiple places

1. Once for loading the index JSON files
2. Once for signing the reference filesystem templates

Which lets us replace this:

>>> result = xr.open_dataset(
...     fsspec.get_mapper(
...         "reference://",
...         fo=planetary_computer.sign(requests.get(planetary_computer.sign("https://deltaresreservoirssa.blob.core.windows.net/references/reservoirs/chirps.json")).json()),
...     ),
...     engine="zarr",
...     consolidated=False,
... )

With this:

>>> result = xr.open_dataset(
...     "pc::reference::pc::https://deltaresreservoirssa.blob.core.windows.net/references/reservoirs/CHIRPS.json",
...     engine="zarr",
...     consolidated=False,
... )

Still just a POC. I need to figure out

1. Better tests.
2. If there's a way to modifier the references earlier.

[TomAugspurger]

I'm not a fan of this block.

The reference filesystem has the idea of "template" URLs, which are the NetCDF files in blob storage. We want to sign those URLs before anyone attempts to access data via this reference filesystem.

It seems that the reference filesystem's __init__ calls a method at https://github.com/fsspec/filesystem_spec/blob/7effb83e8ab31010ec5796c14193b5fcd5774e05/fsspec/implementations/reference.py#L149, which does a lot of work including in-lining the template URLs in the reference (url, start, end) tuples. Unfortunately, we don't have a way to update the template URLs before the tuples are built, so we have to do it again.