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GH-35975: [Go] Support importing decimal256 (#35981)

### Rationale for this change

We didn't implement this type.

### What changes are included in this PR?

We now import this type. (And handle some other error cases.)

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* Closes: #35975

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>