The "Init Duration" reported by the RIE is wrong

The "Init Duration" reported by the RIE is much too small. It is taken [before the actual language runtime is even started](https://github.com/aws/aws-lambda-runtime-interface-emulator/blob/2057adac54dbd781b51431f775363ec021648c77/cmd/aws-lambda-rie/handlers.go#L98):
```go
initStart, initEnd := InitHandler(sandbox, functionVersion, timeout, bs)
```

Instead, the end of the Init phase should be recorded when the [`sendInitRuntimeDoneLogEvent()`](https://github.com/aws/aws-lambda-runtime-interface-emulator/blob/2057adac54dbd781b51431f775363ec021648c77/lambda/rapid/handlers.go#L358) is posted:
```go
defer func() {
sendInitRuntimeDoneLogEvent(execCtx, sbInfoFromInit.SandboxType, runtimeDoneStatus)
}()
```

从 cmd/aws-lambda-rie/handlers.go 中的 InitHandler 调用开始，然后跟踪 init 生命周期，直到 lambda/rapid/handlers.go 和 sendInitRuntimeDoneLogEvent()。验证 Init 阶段何时结束，并确保报告的 Init Duration 包含 language runtime 启动时间；将生成的 RIE 日志事件与预期 timing 进行比较。

由索引模型根据 Issue 内容生成。