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/APIProtocol.md

@@ -95,3 +95,10 @@
 - ``APIProtocol/updateInWantlist(path:query:headers:)``
 - ``APIProtocol/deleteFromWantlist(_:)``
 - ``APIProtocol/deleteFromWantlist(path:headers:)``
+
+### User Lists
+
+- ``APIProtocol/getLists(_:)``
+- ``APIProtocol/getLists(path:query:headers:)``
+- ``APIProtocol/getList(_:)``
+- ``APIProtocol/getList(path:headers:)``