feat(lambda-events): add Control Tower lifecycle events module

Add a `controltower` module to `aws_lambda_events` covering all 13 AWS
Control Tower lifecycle events delivered via EventBridge as CloudTrail
service events.

Events covered:
- CreateManagedAccount / UpdateManagedAccount
- EnableGuardrail / DisableGuardrail
- SetupLandingZone / UpdateLandingZone
- RegisterOrganizationalUnit / DeregisterOrganizationalUnit
- PrecheckOrganizationalUnit
- EnableBaseline / ResetEnabledBaseline / UpdateEnabledBaseline / DisableBaseline

The `ServiceEventDetails` enum uses serde's externally-tagged
representation to match the `serviceEventDetails` JSON structure where
the key name identifies the event type.

Usage with the CloudWatchEvent or EventBridgeEvent envelope:

```
    let event: CloudWatchEvent<ControlTowerLifecycleEvent> =
        serde_json::from_slice(data)?;
```

Author: martinjlowm

lambda-events/Cargo.toml

@@ -79,6 +79,7 @@ default = [
   "streams",
   "documentdb",
   "eventbridge",
+  "controltower",
   "vpc_lattice"
 ]
 
@@ -103,6 +104,7 @@ codepipeline_job = []
 cognito = []
 config = []
 connect = []
+controltower = []
 dynamodb = ["chrono", "serde_dynamo", "streams"]
 ecr_scan = []
 firehose = ["chrono"]