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GH-35711: [Go] Add `Value` and `GetValueIndex` methods to some builders (#35744)

### Rationale for this change

See #35711 

### What changes are included in this PR?

Adding `*BinaryDictionaryBuilder.GetValueIndex(i int) int` to get the value's index in the dictionary. Then two methods to get a `[]byte` and `string` results. `Value(i int) []byte` and `ValueStr(i int) string`.

### Are these changes tested?

So far I have added very little testing. Mostly some sort of integration test to read back values from the BinaryDictionaryBuilder. We should definitely add some test for the `*BinaryMemoTable) Value(i int) []byte`. 

### Are there any user-facing changes?

Just additions, no breaking API changes. 

cc @ zeroshade 
* Closes: #35711

Authored-by: Matthias Loibl <mail@matthiasloibl.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>