All struct data types

[andreaTP]

Implement specific data accessors for all the types of Schema.

[jpechane]

@andreaTP LGTM, I think we need setters for all types too.
Also in a separate PR it would be nice to handle ARRAY as well, but we can keep it focused on primitive types now.

[andreaTP]

I think we need setters for all types too

I thought about it, but, for the current use case only bool, string and nulls are necessary as far as I understand it.
The setter are needed to return a value to the Java Host:

• filter: boolean or null
• router: string or null

am I missing a use case?