(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/common/AuditEvent.ts

@@ -0,0 +1,31 @@
+export interface AuditEvent<Name extends AuditEventName> {
+  event: {
+    /** The event name. */
+    name: Name;
+    /** The user that triggered the event. */
+    user: AuditEventUser | null;
+    /** Additional event details. */
+    details: AuditEventDetails[Name] | null;
+  };
+  /** ISO 8601 timestamp of when the event was logged. */
+  timestamp: string;
+}
+
+export type AuditEventName =
+  | 'createDocument';
+
+export interface AuditEventUser {
+  /** The user's id. */
+  id: number | null;
+  /** The user's email address. */
+  email: string | null;
+  /** The user's name. */
+  name: string | null;
+}
+
+export interface AuditEventDetails {
+  createDocument: {
+    /** The ID of the document. */
+    id: string;
+  };
+}