add an endpoint for doing SQL selects (#641)

* add an endpoint for doing SQL selects

This adds an endpoint for doing SQL selects directly on a Grist document. Other kinds of statements are not supported. There is a default timeout of a second on queries.

This follows loosely an API design by Alex Hall.

Co-authored-by: jarek <jaroslaw.sadzinski@gmail.com>

app/gen-server/lib/DocApiForwarder.ts

@@ -61,6 +61,7 @@ export class DocApiForwarder {
     app.use('/api/docs/:docId/webhooks/queue', withDoc);
     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$', withoutDoc);
   }