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
3e22b89fa2