feat: allow multipart requests for public api

Adds support for multipart/form-data requests in the public api
allowing documents to be uploaded without having to perform a secondary
request.

Need to rollout further endpoints for envelopes and templates.

Need to change how we store files to not use `putFileServerSide`

packages/trpc/utils/data-transformer.ts

@@ -0,0 +1,15 @@
+import type { DataTransformer } from '@trpc/server';
+import SuperJSON from 'superjson';
+
+export const dataTransformer: DataTransformer = {
+  serialize: (data: unknown) => {
+    if (data instanceof FormData) {
+      return data;
+    }
+
+    return SuperJSON.serialize(data);
+  },
+  deserialize: (data: unknown) => {
+    return SuperJSON.deserialize(data);
+  },
+};