(core) control the distribution of attachment metadata

Summary:
for users who don't automatically have deep rights
to the document, provide them with attachment metadata only
for rows they have access to. This is a little tricky to
do efficiently. We provide attachment metadata when an
individual table is fetched, rather than on initial document
load, so we don't block that load on a full document scan.
We provide attachment metadata to a client when we see that
we are shipping rows mentioning particular attachments,
without making any effort to keep track of the metadata they
already have.

Test Plan: updated tests

Reviewers: dsagal, jarek

Reviewed By: dsagal, jarek

Differential Revision: https://phab.getgrist.com/D3722

app/server/lib/DocApi.ts

@@ -164,7 +164,7 @@ export class DocWorkerApi {
       }
       const tableId = optTableId || req.params.tableId;
       const session = docSessionFromRequest(req);
-      const tableData = await handleSandboxError(tableId, [], activeDoc.fetchQuery(
+      const {tableData} = await handleSandboxError(tableId, [], activeDoc.fetchQuery(
         session, {tableId, filters}, !immediate));
       // For metaTables we don't need to specify columns, search will infer it from the sort expression.
       const isMetaTable = tableId.startsWith('_grist');