aws-lambda: Cannot use AdotInstrumentation for dotnet lambdas

### Describe the bug

Accodring to the [Getting Started](https://aws-otel.github.io/docs/getting-started/lambda/lambda-dotnet) page, you don't need to set ExecWrapper for dotnet lambdas.
However, `ExecWrapper` is required with CDK.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Expected Behavior

AdotInstrumentation should allow to omit ExecWrapper property.

### Current Behavior

ExecWrapper property is mandatory.

### Reproduction Steps

```csharp
new Function(this, "func-id", new FunctionProps
{
Handler = "My::Handler::Method",
Code = Code.FromAsset("publish/MyCode.zip"),
Runtime = Runtime.DOTNET_8,
AdotInstrumentation = new AdotInstrumentationConfig
{
LayerVersion = AdotLayerVersion.FromGenericLayerVersion(AdotLambdaLayerGenericVersion.LATEST)
}
});

// SII001 The property ExecWrapper is required and cannot be null.
```

### Language

.NET

No source file or test is named. Start by reproducing the provided CDK FunctionProps example with a .NET 8 Lambda and trace the AdotInstrumentation validation; done means the configuration succeeds without setting ExecWrapper, with coverage for this case.

Written by the indexing model from the issue text.