feat(medusa): Add feature flags to store response (#1819)

**What**
- Add `feature_flags` string array to store response

**Why**
- to provide conditional ui in admin corresponding to enabled features

Co-authored-by: Oliver Windall Juhl <59018053+olivermrbl@users.noreply.github.com>

integration-tests/api/__tests__/admin/__snapshots__/store.js.snap

@@ -121,6 +121,12 @@ Object {
     "symbol_native": "$",
   },
   "default_currency_code": "usd",
+  "feature_flags": Array [
+    Object {
+      "key": "sales_channels",
+      "value": false,
+    },
+  ],
   "fulfillment_providers": Array [
     Object {
       "id": "test-ful",

integration-tests/api/__tests__/admin/store.js

@@ -52,6 +52,12 @@ describe("/admin/store", () => {
             code: "usd",
           },
         ],
+        feature_flags: [
+          {
+            key: "sales_channels",
+            value: false,
+          },
+        ],
         default_currency_code: "usd",
         created_at: expect.any(String),
         updated_at: expect.any(String),