Provide and interface to query and fragments

[ap--]

We need an interface to uri query parameters and fragments.

Could be:

class UPath(...):

    @property
    def query(self) -> Mapping[str, str]:
        ...

    @property
    def fragment(self) -> str:
        ...

    def with_query(self, **params: Any) -> UPath:
        ...

    def with_fragment(self, fragment: str) -> UPath:
        ...

[ap--]

Once this is implemented we can deprecate ._url