[Backend] Set audit event HTTP destination

Proposal

To enable GitLab to send audit events to an external HTTP desintation, we need to allow root-namespace owners to set a destination URL that will apply to all group and project level events within that namespace.

Implementation Plan

• Create new feature flag for the entire epic and rollout issue. (:group_emit_external_audit_events)
• Create new model: ExternalAuditEventDestination
  • It's likely that whilst this first iteration will be a 1:1 relationship between namespace:destination, it will probably become a 1:N relationship.
  • Namespace has_many ExternalAuditEventDestinations but the UI will infer that only one is available right now.
• Create GraphQL mutation to allow Owners to set this value. A blank value is valid.