Add storage options and protocol

[ap--]

This PR adds .storage_options and .protocol to UPath.

The "non-pathlib" interface of UPath is then:

• UPath.protocol: str the fsspec protocol
• UPath.storage_options: dict[str, Any] the storage_options for the fsspec class
• UPath.path: str the fsspec compatible path for use with the fsspec class
• UPath.fs: AbstractFileSystem convenience attribute to access a instantiated fsspec class

the first three provide a public interface to access a file via fsspec as follows:

pth = UPath(..., **sopts)

from fsspec import filesystem

fs = filesystem(pth.protocol, **pth.storage_options)
with fs.open(pth.path) as f:
    data = f.read()

This closes #129, and closes #91.

additional changes:

• To support correct behavior for cloud filesystems .path had to be overwritten to return a path that includes the bucket (consistent with fsspec).
• Since the webdav filesystem doesn't serialize to string well (which is why we use the webdav+http and webdav+https protocol workaround) the WebdavPath.protocol returns "webdav" and the base_url is added to the .storage_options. Which then allows to support the usecase from the code mentioned above again.
• HTTPPath().path returns a full url, in line with fsspec behavior.

extra note regarding local filesystems

# protocol is empty string for non-uri localpaths:
p = UPath("/path/file.txt")
assert isinstance(p, PosixUPath)
assert p.protocol == ""

# and it's "file" for uri localpaths
u = UPath("file:///path/file.txt")
assert isinstance(p, LocalPath)
assert p.protocol == "file"

Both "file" and "" return LocalFileSystem when using fsspec.get_filesystem_class(protocol).