feat: Sales Channels API routes + workflows (#6722)

**What**
- Admin sales channels API routes
- Workflows for creating, updating, and deleting sales channels
- Align `ISalesChannelModuleService` interface with update + upsert conventions
- Integrate v2 tests into v1 sales channels integration test suite
- Add metadata to sales channel entity

Sales channel <> Product management will come in a follow-up PR.

On the integration tests, creating, updating, and deleting are passing with the V2 flag enabled. You'll have to take my word for it (or run them yourself), as they are not included in the CI.

integration-tests/api/__tests__/admin/__snapshots__/sales-channels.js.snap

@@ -1,6 +1,6 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`sales channels DELETE /admin/sales-channels/:id should delete the requested sales channel 1`] = `
+exports[` DELETE /admin/sales-channels/:id should delete the requested sales channel 1`] = `
 Object {
   "deleted": true,
   "id": Any<String>,
@@ -8,7 +8,7 @@ Object {
 }
 `;
 
-exports[`sales channels POST /admin/sales-channels successfully creates a disabled sales channel 1`] = `
+exports[` POST /admin/sales-channels successfully creates a disabled sales channel 1`] = `
 Object {
   "sales_channel": ObjectContaining {
     "is_disabled": true,
@@ -17,7 +17,7 @@ Object {
 }
 `;
 
-exports[`sales channels POST /admin/sales-channels successfully creates a sales channel 1`] = `
+exports[` POST /admin/sales-channels successfully creates a sales channel 1`] = `
 Object {
   "sales_channel": ObjectContaining {
     "description": "sales channel description",