Implemented the Marketplace endpoints (#15)

This PR contains the work done to implement the *Marketplace* endpoints for the **Discogs API** backend service, which include the following endpoints:
* GET `/users/{username}/inventory` endpoint
* GET `/marketplace/listings/{listing_id}` endpoint
* DELETE `/marketplace/listings/{listing_id}` endpoint
* POST `/marketplace/listings/{listing_id}` endpoint
* POST `/marketplace/listings` endpoint
* GET `/marketplace/orders` endpoint
* GET `/marketplace/orders/{order_id}` endpoint
* GET `/marketplace/orders/{order_id}/messages` endpoint
* POST `/marketplace/orders/{order_id}/messages` endpoint
* GET `/marketplace/fee/{price}` endpoint
* GET `/marketplace/fee/{price}/{currency}` endpoint
* GET `/marketplace/price_suggestions/{release_id}` endpoint
* GET `/marketplace/stats/{release_id}` endpoint

Reviewed-on: #15
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

@@ -65,4 +65,20 @@
 ### User Lists
 
 - ``Client/getLists(_:)``
-- ``Client/getList(_:)``
+- ``Client/getList(_:)``
+
+### Marketplace
+
+- ``Client/getInventory(_:)``
+- ``Client/getListing(_:)``
+- ``Client/createListing(_:)``
+- ``Client/editListing(_:)``
+- ``Client/deleteListing(_:)``
+- ``Client/getOrders(_:)``
+- ``Client/getOrder(_:)``
+- ``Client/getOrderMessages(_:)``
+- ``Client/addOrderMessage(_:)``
+- ``Client/getFee(_:)``
+- ``Client/getFeeWithCurrency(_:)``
+- ``Client/getPriceSuggestions(_:)``
+- ``Client/getStatistics(_:)``