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GH-37416: [Go] Allow accessing underlying index builder of dictionary builders (#37417)

### Rationale for this change

See #37416 

### What changes are included in this PR?

Exposing the index builder of dictionary builders.

### Are these changes tested?

Tested by an external usage. Since this is about exporting something that seemed appropriate enough.

### Are there any user-facing changes?

A new API, so no.
* Closes: #37416

Authored-by: Frederic Branczyk <fbranczyk@gmail.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>