feat(iam): add SetUserStatus and UpdateAccessKey actions (#7745)
Add ability to enable/disable users and access keys without deleting them.
## Changes
### Protocol Buffer Updates
- Add `disabled` field (bool) to Identity message for user status
- false (default) = enabled, true = disabled
- No backward compatibility hack needed since zero value is correct
- Add `status` field (string: Active/Inactive) to Credential message
### New IAM Actions
- SetUserStatus: Enable or disable a user (requires admin)
- UpdateAccessKey: Change access key status (self-service or admin)
### Behavior
- Disabled users: All API requests return AccessDenied
- Inactive access keys: Signature validation fails
- Status check happens early in auth flow for performance
- Backward compatible: existing configs default to enabled (disabled=false)
### Use Cases
1. Temporary suspension: Disable user access during investigation
2. Key rotation: Deactivate old key before deletion
3. Offboarding: Disable rather than delete for audit purposes
4. Emergency response: Quickly disable compromised credentials
Fixes#7745
