(core) Forms feature

Summary:
A new widget type Forms. For now hidden behind GRIST_EXPERIMENTAL_PLUGINS().
This diff contains all the core moving parts as a serves as a base to extend this functionality
further.

Test Plan: New test added

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D4130

app/gen-server/lib/DocApiForwarder.ts

@@ -62,6 +62,7 @@ export class DocApiForwarder {
     app.use('/api/docs/:docId/webhooks', withDoc);
     app.use('/api/docs/:docId/assistant', withDoc);
     app.use('/api/docs/:docId/sql', withDoc);
+    app.use('/api/docs/:docId/forms/:id', withDoc);
     app.use('^/api/docs$', withoutDoc);
   }