generate random struct and other data types

[lmatz]

Is your feature request related to a problem? Please describe.

Currently, for example, we declare a source with struct as one of the fields and the connector = datagen. create source will succeed.

And only when we create a materialized on top of the source, does the FieldGeneratorImpl complain that this is not implemented yet.

Describe the solution you'd like

We just implement a random data generator for struct type.

Describe alternatives you've considered

No response

Additional context

There are a couple of other data types that can not be generated now.