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GH-34171: [Go][Compute] Implement "Unique" kernel (#34172)



### Rationale for this change

Implementing a kernel for computing the "unique" values in an arrow array, primarily for use in solving #33466. 

### What changes are included in this PR?
Adds a "unique" function to the compute list and helper convenience functions.

### Are these changes tested?
Yes, unit tests are included.

### Are there any user-facing changes?
Just the new available functions.

* Closes: #34171

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