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GH-37694: [Go] Add SetNull to array builders (#37695)



### Rationale for this change

We are proposing to add a `SetNull(i int)` to the `Builder` interface.
The underlying `builder` type implements this for most types.

### What changes are included in this PR?

### Are these changes tested?

### Are there any user-facing changes?

The SetNull is added to the Builders type.

* Closes: #37694

Authored-by: Matthias Loibl <mail@matthiasloibl.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>