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GH-41140: [C#] Account for offset and length in union arrays (#41165)

### Rationale for this change

See #41140. This makes a sliced union array behave as expected without having to manually account for the array offset unless accessing the underlying buffers.

### What changes are included in this PR?

Accounts for the offset and length when getting type ids, value offsets and field arrays for sparse and dense union arrays.

### Are these changes tested?

Yes, I've updated the union array tests to cover this.

### Are there any user-facing changes?

Yes, this is a user facing bug fix.
* GitHub Issue: #41140

Authored-by: Adam Reeve <adreeve@gmail.com>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>