Audit key destroy action

What does this MR do?

Audit successful key destroy action.

When an SSH key is successfully destroyed, an audit event will be created. Additionally, we introduce a domain object Audit::Target to handle a more flexible resolution of the target's details. Previously, it is assumed that all audit target has name but this is no longer valid for Key (i.e. Key has title).

How to test

1. Authenticated to the RESTFul API, and execute the following curl command

# As a normal user
curl -i -H Authorization\:\ Bearer\ y2SAvz9c2nPRcg6FjxWV -XDELETE http\://gdk.test\:3000/api/v4/user/keys/13

# As an admin user
curl -i -H Authorization\:\ Bearer\ y2SAvz9c2nPRcg6FjxWV -XDELETE http\://gdk.test\:3000/api/v4/users/123/keys/13

2. Navigate to Admin Area > Monitoring > Audit Events and view the newly created audit events

Screenshots or Screencasts (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Related to #220127 (closed)