Add ability to create a copy constructor

[c-mansfield]

This PR adds the ability for a copy constructor as seen below.

` public static final class RecordABuilder {
private String fieldOne;
private String fieldTwo;

    private RecordABuilder() {
    }

    private RecordABuilder(RecordA recordA) {
        this.fieldOne = recordA.fieldOne();
        this.fieldTwo = recordA.fieldTwo();
    }
 ...

}`

Currently an optional feature through the ui popup.

[c-mansfield]

Hey @mjedynak, when you get a minute could you please take a look at this PR 😃

[mjedynak]

hi @c-mansfield, thanks for the PR, I'll try to look at it shortly.

[mjedynak]

I cherry picked your commits as there were some issues (generated code was not compiling in case of using single field and copy constructor) that I fixed myself.
Released in 1.4.0.

[c-mansfield]

Thank you @mjedynak