feat(orders): add Approvals, ShipmentConfirmation

Add new Amazon API changes of the orders-api and minor refactorings.

Changelog:

* fix: installation url in README.md

* feat: add Set in internal utils pkg

* feat: add approvals and shipmentConfirmation to OrdersAPI

* feat: add new types in orders model.go

* refactor: use enums in orders api with sets

* fix(orders)!: add missing callResponse return
  Applies to
  + `UpdateOrderItemsApprovals`
  + `ConfirmShipment`

README.md

@@ -5,7 +5,7 @@ Golang client implementation to the [Amazon SP-API](https://developer-docs.amazo
 ## Installation
 
 ```shell
-go get -u https://github.com/fond-of-vertigo/amazon-sp-api
+go get -u github.com/fond-of-vertigo/amazon-sp-api
 ```
 
 ## API-Endpoints coverage

apis/feeds/model.go

@@ -1,6 +1,7 @@
 package feeds
 
 import (
+	"github.com/fond-of-vertigo/amazon-sp-api/internal/utils"
 	"net/url"
 	"strconv"
 	"strings"
@@ -100,13 +101,13 @@ type GetFeedsRequestFilter struct {
 func (f *GetFeedsRequestFilter) GetQuery() url.Values {
 	q := url.Values{}
 
-	feedTypes := strings.Join(apis.FirstNElementsOfSlice(f.FeedTypes, 10), ",")
+	feedTypes := strings.Join(utils.FirstNElementsOfSlice(f.FeedTypes, 10), ",")
 	if feedTypes != "" {
 		q.Set("feedTypes", feedTypes)
 	}
 
-	topTenMarketplaceIDs := apis.FirstNElementsOfSlice(f.MarketplaceIDs, 10)
-	marketplaceIds := apis.MapToCommaString(topTenMarketplaceIDs)
+	topTenMarketplaceIDs := utils.FirstNElementsOfSlice(f.MarketplaceIDs, 10)
+	marketplaceIds := utils.MapToCommaString(topTenMarketplaceIDs)
 	if marketplaceIds != "" {
 		q.Set("marketplaceIds", marketplaceIds)
 	}