feat: Create payment sessions (#6549)

~~Opening a draft PR to discuss a couple of implementation details that we should align on~~

**What**

Add workflow and API endpoint for creating payment sessions for a payment collection. Endpoint is currently `POST /store/payment-collection/:id/payment-sessions`. I suggested an alternative in a comment below.

Please note, we intentionally do not want to support creating payment sessions in bulk, as this would become a mess when having to manage multiple calls to third-party providers.

packages/payment/integration-tests/__fixtures__/data.ts

@@ -25,21 +25,21 @@ export const defaultPaymentSessionData = [
     amount: 100,
     currency_code: "usd",
     provider_id: "pp_system_default",
-    payment_collection: "pay-col-id-1",
+    payment_collection_id: "pay-col-id-1",
   },
   {
     id: "pay-sess-id-2",
     amount: 100,
     currency_code: "usd",
     provider_id: "pp_system_default",
-    payment_collection: "pay-col-id-2",
+    payment_collection_id: "pay-col-id-2",
   },
   {
     id: "pay-sess-id-3",
     amount: 100,
     currency_code: "usd",
     provider_id: "pp_system_default",
-    payment_collection: "pay-col-id-2",
+    payment_collection_id: "pay-col-id-2",
   },
 ]
 
@@ -48,7 +48,7 @@ export const defaultPaymentData = [
     id: "pay-id-1",
     amount: 100,
     currency_code: "usd",
-    payment_collection: "pay-col-id-1",
+    payment_collection_id: "pay-col-id-1",
     payment_session: "pay-sess-id-1",
     provider_id: "pp_system_default",
     authorized_amount: 100,
@@ -59,7 +59,7 @@ export const defaultPaymentData = [
     amount: 100,
     authorized_amount: 100,
     currency_code: "usd",
-    payment_collection: "pay-col-id-2",
+    payment_collection_id: "pay-col-id-2",
     payment_session: "pay-sess-id-2",
     provider_id: "pp_system_default",
     data: {},