wip: start implementing chain functionality

[ap--]

Draft PR for starting implementation for fsspec urlpath chain support.

• UPath should start relying on CurrentChainSegment to store path, protocol, and storage_options.
• Need to think through if by default, the target protocol should be the first or last protocol used in the chain
• There needs to be some way to select which chain segment UPath will operate on

  >>> p0 = UPath("zip://abc/efg.csv::s3://bucket/file.zip")
  >>> p0
  S3Path("zip://abc/efg.csv::s3://bucket/file.zip")
  >>> p1 = p0.chain[-2]  # maybe this?
  >>> p1 = p0.chain_target("zip")  # or maybe this?
  >>> p1
  ZipPath("zip://abc/efg.csv::s3://bucket/file.zip")

• We need to figure out how filesystems that just pass the path through show up. For example SimpleCache path semantics should adhere to the downstream path semantics... (ChainSegment.path can be None for detecting these cases)