Allow selecting urlpath chain segment

[ap--]

There needs to be some way to select which chain segment UPath will operate on. Need to think through how this will work.

>>> 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")