Implemented the Authentication endpoints (#9)

This PR contains the work done to implement the *Authentication* endpoints of the Discogs API:
* GET `/oauth/request_token`
* POST `/oauth/access_token`
* GET `/oauth/identity`

Reviewed-on: #9
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

@@ -7,6 +7,15 @@
 - ``APIProtocol/getService(_:)``
 - ``APIProtocol/getService(headers:)``
 
+### Authentication endpoints
+
+- ``APIProtocol/getRequestToken(_:)``
+- ``APIProtocol/getRequestToken(headers:)``
+- ``APIProtocol/postAccessToken(_:)``
+- ``APIProtocol/postAccessToken(headers:)``
+- ``APIProtocol/getUserIdentity(_:)``
+- ``APIProtocol/getUserIdentity(headers:)``
+
 ### Database endpoints
 
 - ``APIProtocol/searchDatabase(_:)``

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

@@ -10,6 +10,12 @@
 
 - ``Client/getService(_:)``
 
+### Authentication endpoints
+
+- ``Client/getRequestToken(_:)``
+- ``Client/postAccessToken(_:)``
+- ``Client/getUserIdentity(_:)``
+
 ### Database endpoints
 
 - ``Client/searchDatabase(_:)``