Add support for union of interfaces

Right now only primitive types are supported as unions.

What would support for interfaces look like?

https://github.com/cdklabs/json2jsii/blob/b6091c8d9d33a200f03811a8e819cc600c78c5b5/src/type-generator.ts#L576-L578

https://github.com/cdklabs/json2jsii/blob/b6091c8d9d33a200f03811a8e819cc600c78c5b5/src/type-generator.ts#L350-L352

Start by reading src/type-generator.ts at the linked locations around lines 576-578 and 350-352 to understand the existing primitive-union handling. Determine the intended behavior for unions of interfaces before identifying the implementation and validation work needed; done means interface unions are supported consistently with the project’s jsii-compatible output.

Written by the indexing model from the issue text.