(core) Add audit logging machinery

Summary:
Adds machinery to support audit logging in the backend.

Logging is currently implemented by streaming events to external HTTP
endpoints. All flavors of Grist support a default "grist" payload format,
and Grist Enterprise additionally supports an HEC-compatible payload format.

Logging of all audit events will be added at a later date.

Test Plan: Server tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D4331

app/gen-server/ApiServer.ts

@@ -283,6 +283,12 @@ export class ApiServer {
           altSessionId: mreq.altSessionId,
         },
       });
+      this._gristServer.getAuditLogger().logEvent(mreq, {
+        event: {
+          name: 'createDocument',
+          details: {id: docId},
+        },
+      });
       return sendReply(req, res, query);
     }));