Add Postmark endpoint and initial collection configuration

endpoints/[POST] postmark.bru

@@ -0,0 +1,11 @@
+meta {
+  name: [POST] postmark
+  type: http
+  seq: 2
+}
+
+post {
+  url: https://inbox-negotiator.vercel.app/api/postmark
+  body: none
+  auth: none
+}