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.

yarn.lock

@@ -8642,6 +8642,7 @@ __metadata:
     "@medusajs/modules-sdk": ^1.12.5
     "@medusajs/types": ^1.11.9
     "@medusajs/utils": ^1.11.2
+    "@medusajs/workflows-sdk": "workspace:^"
     "@mikro-orm/cli": 5.9.7
     "@mikro-orm/core": 5.9.7
     "@mikro-orm/migrations": 5.9.7
@@ -9126,7 +9127,7 @@ __metadata:
   languageName: unknown
   linkType: soft
 
-"@medusajs/workflows-sdk@^0.1.2, @medusajs/workflows-sdk@^0.1.3, @medusajs/workflows-sdk@workspace:packages/workflows-sdk":
+"@medusajs/workflows-sdk@^0.1.2, @medusajs/workflows-sdk@^0.1.3, @medusajs/workflows-sdk@workspace:^, @medusajs/workflows-sdk@workspace:packages/workflows-sdk":
   version: 0.0.0-use.local
   resolution: "@medusajs/workflows-sdk@workspace:packages/workflows-sdk"
   dependencies: