Populate gzip context

[effigies]

If a file suffix is .gz, it should populate a gzip context.

The object needs to follow the protocol bidsschematools.types.protocols.Gzip, possibly just using bidsschematools.types.context.Gzip.

The Python stdlib does not provide access to the fields we look for, so it is probably simplest to follow the lead of the typescript validator: https://github.com/bids-standard/bids-validator/blob/cf494fd/src/files/gzip.ts

def read_gzip_header(file: FileTree) -> bst.types.context.Gzip:
    with open(file, 'rb') as fobj:
        buffer = fobj.read(512)
    ...
    return Gzip(timestamp, filename, comment)