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GH-35965: [Go] Fix `Decimal256DictionaryBuilder` (#35966)



### Rationale for this change
`Decimal256DictionaryBuilder` cannot append `decimal256` value and prefers `decimal128` instead.
```
cannot use v (variable of type decimal256.Num) as decimal128.Num value in argument to builder.Append
```

### What changes are included in this PR?

### Are these changes tested?
Yes

### Are there any user-facing changes?
Yes

* Closes: #35965

Authored-by: izveigor <izveigor@gmail.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>