Implemented the User Lists endpoints (#14)

This PR contains the work done to implement the *User Lists* endpoints of the *Discogs API* service in the `Open API` specification document:

* GET `/users/{username}/lists`
* GET `/lists/{list_id}`

Reviewed-on: #14
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Sources/DiscogsService/Catalogs/DiscogsService.docc/Extensions/Client.md

@@ -61,3 +61,8 @@
 - ``Client/addToWantlist(_:)``
 - ``Client/updateInWantlist(_:)``
 - ``Client/deleteFromWantlist(_:)``
+
+### User Lists
+
+- ``Client/getLists(_:)``
+- ``Client/getList(_:)``