Structs + type unions don't work in Java

## :bug: Bug Report

### Affected Languages

- [ ] `TypeScript` or `Javascript`
- [ ] `Python`
- [x] `Java`
- [ ] .NET (`C#`, `F#`, ...)
- [ ] `Go`

### General Information
* **JSII Version:** 1.29.0
* **Platform:** Darwin 186590cdb71d.ant.amazon.com 18.7.0 Darwin Kernel Version 18.7.0: Mon Mar 8 22:11:48 PST 2021; root:xnu-4903.278.65~1/RELEASE_X86_64 x86_64

### What is the problem?

Structs that have type unions do not work correctly in Java.

For example:

```java
CfnBucket cfnBucket = CfnBucket.Builder.create(this, "CfnBucket").build();
cfnBucket.setAccelerateConfiguration(CfnBucket.AccelerateConfigurationProperty.builder()
.accelerationStatus("Suspended")
.build());
System.out.println(cfnBucket.getAccelerateConfiguration());
```

prints out:

```
software.amazon.jsii.JsiiObject@68a13400
```

while it clearly should return an instance of `CfnBucket.AccelerateConfigurationProperty`.

This is not a problem in other languages; for example, in Python:

```python
bucket = s3.CfnBucket(
self, 'CfnBucket'
)
bucket.accelerate_configuration = s3.CfnBucket.AccelerateConfigurationProperty(
acceleration_status='Suspended',
)
print(bucket.accelerate_configuration)
```

prints out:

```
AccelerateConfigurationProperty(acceleration_status='Suspended')
```

Which is what I would expect.

This is negatively impacting CDK customers that use Java (example: https://github.com/aws/aws-cdk/issues/12338).

Start by reproducing the Java CfnBucket.AccelerateConfigurationProperty example and comparing its getter output with the Python example. Trace how jsii represents and returns Java structs containing type unions; the issue is done when the Java getter returns the expected AccelerateConfigurationProperty instance instead of a generic JsiiObject.

Written by the indexing model from the issue text.