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>

docs/data/documentation/discogsservice/authtransport/!=(_:_:).json

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