support Fn::ToJsonString

It seems like the new ToJsonString intrinsic function is not yet available in Troposphere:
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-ToJsonString.html

An example use-case for the function is to be able to use the return value "OpenWireEndpoints" of AWS:AmazonMQ:Broker as an environment variable for an Elastic Beanstalk environment:
```python
OptionSetting(
OptionName="ACTIVE_MQ_ENDPOINTS",
Namespace="aws:elasticbeanstalk:application:environment",
Value=ToJsonString(GetAtt(broker, "OpenWireEndpoints"))
),
```

Start with Troposphere's existing intrinsic-function support and the linked AWS ToJsonString reference. Trace how GetAtt values are represented and locate the related tests; done means the documented ToJsonString example can be represented and the tests pass.

Written by the indexing model from the issue text.