fetch2: Add a class representing a generic URI

fetch2: Add a class representing a generic URI

A class representing a generic URI, with methods for accessing the URI
components, and stringifies to the URI. This class should be a bit more
flexible than the existing {encode,decode}_url functions in that it
supports more components (e.g. port) and that it does not rely on a
specific order on the return values. This makes it easy to add new
properties without affecting the API.

Signed-off-by: Olof Johansson <olof.johansson@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>