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MINOR: [Go] Export IndexBuilder struct (#37438)

We allow accessing the IndexBuilder since GH-37416 merged. While the `Builder` interface functions can be accessed, the actual array builder cannot.

### Rationale for this change

#37416 added the function to access the index builder, but since it returns the `Builder` interface, we can't actually access the underlying builder as the unexported `IndexBuilder` is unexported.

Alternatives could be:

* Return `indexBuilder.Builder` instead in the `IndexBuilder()` function
* Return the unexported `indexBuilder` type, but this tends to be more awkward to use

### What changes are included in this PR?

Exporting the type so it can be type asserted.

### Are these changes tested?

Yes, rolled this out in our infra.

### Are there any user-facing changes?

Yes, but this API hasn't been released yet.

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