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/APIProtocol.md

@@ -45,7 +45,7 @@
 - ``APIProtocol/getReleaseStats(_:)``
 - ``APIProtocol/getReleaseStats(path:headers:)``
 
-### User Identity
+### User Identity endpoints
 
 - ``APIProtocol/getUserProfile(_:)``
 - ``APIProtocol/getUserProfile(path:headers:)``
@@ -56,7 +56,7 @@
 - ``APIProtocol/getUserSubmissions(_:)``
 - ``APIProtocol/getUserSubmissions(path:headers:)``
 
-### User Collection
+### User Collection endpoints
 
 - ``APIProtocol/getCollectionFolders(_:)``
 - ``APIProtocol/getCollectionFolders(path:headers:)``
@@ -85,7 +85,7 @@
 - ``APIProtocol/getCollectionValue(_:)``
 - ``APIProtocol/getCollectionValue(path:headers:)``
 
-### User Wantlist
+### User Wantlist endpoints
 
 - ``APIProtocol/getWantlist(_:)``
 - ``APIProtocol/getWantlist(path:query:headers:)``
@@ -96,9 +96,38 @@
 - ``APIProtocol/deleteFromWantlist(_:)``
 - ``APIProtocol/deleteFromWantlist(path:headers:)``
 
-### User Lists
+### User Lists endpoints
 
 - ``APIProtocol/getLists(_:)``
 - ``APIProtocol/getLists(path:query:headers:)``
 - ``APIProtocol/getList(_:)``
-- ``APIProtocol/getList(path:headers:)``
+- ``APIProtocol/getList(path:headers:)``
+
+### Marketplace endpoints
+
+- ``APIProtocol/getInventory(_:)``
+- ``APIProtocol/getInventory(path:query:headers:)``
+- ``APIProtocol/getListing(_:)``
+- ``APIProtocol/getListing(path:query:headers:)``
+- ``APIProtocol/createListing(_:)``
+- ``APIProtocol/createListing(headers:body:)``
+- ``APIProtocol/editListing(_:)``
+- ``APIProtocol/editListing(path:headers:body:)``
+- ``APIProtocol/deleteListing(_:)``
+- ``APIProtocol/deleteListing(path:headers:)``
+- ``APIProtocol/getOrders(_:)``
+- ``APIProtocol/getOrders(query:headers:)``
+- ``APIProtocol/getOrder(_:)``
+- ``APIProtocol/getOrder(path:headers:)``
+- ``APIProtocol/getOrderMessages(_:)``
+- ``APIProtocol/getOrderMessages(path:headers:)``
+- ``APIProtocol/addOrderMessage(_:)``
+- ``APIProtocol/addOrderMessage(path:headers:body:)``
+- ``APIProtocol/getFee(_:)``
+- ``APIProtocol/getFee(path:headers:)``
+- ``APIProtocol/getFeeWithCurrency(_:)``
+- ``APIProtocol/getFeeWithCurrency(path:headers:)``
+- ``APIProtocol/getPriceSuggestions(_:)``
+- ``APIProtocol/getPriceSuggestions(path:headers:)``
+- ``APIProtocol/getStatistics(_:)``
+- ``APIProtocol/getStatistics(path:query:headers:)``

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(_:)``