[FEATURE] Add tenant information to audit logs

[agoerl]

Is your feature request related to a problem?
We configured audit logging and tenancy. Currently it is impossible to audit tenant access/switching due to lack of such information in the audit logging. Access to the wrong tenant is a policy violation for us we cannot currently monitor.

What solution would you like?
Audit logging to tenants, specifically a field "tenant" to indicate to which scope the audit log applies. Also a new event like TENANT_ENTERED would be helpful.

What alternatives have you considered?
We see no alternatives.

Do you have any additional context?
I have no additional context but will happily provide any context if required.

[andrross]

@opensearch-project/admin Can you transfer this to the security repo?

[cwperks]

This was transferred from security-analytics repo.