Adds builder method when using innerBuilder

[josejulio]

This PR updates the code to add a public static method called builder to all the classes that are created with the generateBuilders and useInnerClassBuilders.

This is a sample of how the builder method looks like:

public static Child.ChildBuilderBase builder() {
    return new Child.ChildBuilder();
}

Fixes #1396

[josejulio]

@joelittlejohn I guess I need some approval for the test suite to run.

Let me know if there is anything else to do here, thanks!

[joelittlejohn]

Just closing/opening to trigger checks.