Defined the POST /oauth/access_token endpoints on the OpenAPI specification document in the library target.

Sources/DiscogsService/openapi.yaml

@@ -303,6 +303,36 @@ paths:
               $ref: '#/components/headers/OAuthCallback'
         '500':
           $ref: '#/components/responses/InternalError'
+  /oauth/access_token:
+    post:
+      tags:
+        - Authentication
+      summary: Provide required credentials data to obtain an access token.
+      description: |
+        Provide to the service some required credentials details to obtain an access token at the end of the *OAuth* process.
+
+        This endpoint represents the [4th step of the OAuth flow process](https://www.discogs.com/developers#header-4.-send-a-post-request-to-the-discogs-access-token-url), thus it requires to configure an *Authorization* header to have a value like this:
+        
+        ```
+        OAuth oauth_consumer_key="your_consumer_key", oauth_nonce="random_string_or_timestamp", oauth_token="oauth_token_received_from_step_2" oauth_signature="your_consumer_secret&", oauth_signature_method="PLAINTEXT", oauth_timestamp="current_timestamp", oauth_verifier="users_verifier"
+        ```
+
+        For further details about this process, please refer to the [OAuth flow](https://www.discogs.com/developers#page:authentication,header:authentication-oauth-flow) section in the [Discogs API authentication](https://www.discogs.com/developers#page:authentication) documentation.
+      operationId: postAccessToken
+      parameters:
+        - $ref: '#/components/parameters/ContentType'
+        - $ref: '#/components/parameters/Authorization'
+        - $ref: '#/components/parameters/UserAgent'
+      responses:
+        '200':
+          description: Successfully retrieved an access token at the end of the OAuth authentication process.
+          headers:
+            oauth_token:
+              $ref: '#/components/headers/OAuthToken'
+            oauth_token_secret: 
+              $ref: '#/components/headers/OAuthSecret'
+        '500':
+          $ref: '#/components/responses/InternalError'
   /artists/{artist_id}:
     get:
       tags: