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GH-37584: [Go] Add value len function to string array (#37586)

### Rationale for this change

The binary array already has this function, and it's a useful mechanism when all one needs is the length of a value and not the value itself (for pre-allocation of a new buffer for example).

### What changes are included in this PR?

Add function to return only the length of the string at index position `i`.

### Are these changes tested?

Yes tested by including this in https://parca.dev/

### Are there any user-facing changes?

This is a new API mirroring the same API that already exists on the binary array.
* Closes: #37584

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