Add audit events for product analytics setting changes and external source sharing

For this first iteration of adding audit events to product analytics, we will need audit events for the following activities:

• A new cluster is specified in GitLab for Product Analytics
• An existing cluster is removed from GitLab for Product Analytics
• The project dashboard configuration file source has changed

Implementation plan

Follow the instructions in the Audit Event Guide to create new audit events: https://docs.gitlab.com/ee/development/audit_event_guide

Cluster changes

This is two new events. (change_instance_product_analytics, and change_namespace_product_analytics, or similar)

* Instance-level event when the instance-level settings are changed (this includes, added, deleted or changed).

Application settings are already audited by default. This has been validated in production.

• Project-level event when settings are changed (this includes, added, deleted or changed).

Pointer project changes

• Project-level event when the Custom dashboard projects project-level setting is changed, including when unset.