Implemented the User Wantlist endpoints (#13)

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

* GET `/users/{username}/wants`
* POST `/users/{username}/wants/{release_id}`
* PUT `/users/{username}/wants/{release_id}`
* DELETE `/users/{username}/wants/{release_id}`

Reviewed-on: #13
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

@@ -83,4 +83,15 @@
 - ``APIProtocol/editFieldsInstance(_:)``
 - ``APIProtocol/editFieldsInstance(path:query:headers:)``
 - ``APIProtocol/getCollectionValue(_:)``
-- ``APIProtocol/getCollectionValue(path:headers:)``
+- ``APIProtocol/getCollectionValue(path:headers:)``
+
+### User Wantlist
+
+- ``APIProtocol/getWantlist(_:)``
+- ``APIProtocol/getWantlist(path:query:headers:)``
+- ``APIProtocol/addToWantlist(_:)``
+- ``APIProtocol/addToWantlist(path:query:headers:)``
+- ``APIProtocol/updateInWantlist(_:)``
+- ``APIProtocol/updateInWantlist(path:query:headers:)``
+- ``APIProtocol/deleteFromWantlist(_:)``
+- ``APIProtocol/deleteFromWantlist(path:headers:)``

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

@@ -53,4 +53,11 @@
 - ``Client/deleteInstanceFromCollectionFolder(_:)``
 - ``Client/getCustomFields(_:)``
 - ``Client/editFieldsInstance(_:)``
-- ``Client/getCollectionValue(_:)``
+- ``Client/getCollectionValue(_:)``
+
+### User Wantlist
+
+- ``Client/getWantlist(_:)``
+- ``Client/addToWantlist(_:)``
+- ``Client/updateInWantlist(_:)``
+- ``Client/deleteFromWantlist(_:)``