Register custom compression types via entrypoint mechanism

While new fsspec-compatible filesystem can easily be registered via fsspec.specs entry points, there is no counterpart for compression types.

As we already have the fsspec.compression.register_compression public API for custom compression types, it wouldn't be too much burden (and give better user experience as a universal filesystem interface) to implement entrypoint support for custom compression types.

THOUGHTS.

register_compression takes two names(in addition to the (de-)compressor callable): algorithm name(e.g. gzip) and file extension(e.g. gz). Receiving both names via entrypoint is not impossible (simple parsing in mind) but may not be desirable from aesthetic viewpoint. In any case registration with whatever the preferred name can still be beneficial, as users will be able to specify the compression method via simple string argument, e.g. fsspec.open(..., compression=compression), without import-and-call register_compression manually.

(BTW this feature request comes after my reading a bunch of zlib-compressed csv files. I just found that zlib is not supported in fsspec and can be implemented in almost identical manner as how snappy is implemented.)

Start by reading the existing fsspec.specs entry-point support and the register_compression public API. Resolve how entry points provide the algorithm and file-extension names, then verify that an installed custom compression type can be selected through fsspec.open(..., compression=...). The issue is complete when registration works without manually calling register_compression.