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GH-41199: [C#] Fix accessing values of a sliced decimal array (#41200)

### Rationale for this change

Fixes a bug where getting values from a sliced `Decimal128Array` or `Decimal256Array` would return the wrong values.

### What changes are included in this PR?

Updates the various `Get*` methods in `Decimal128Array` and `Decimal256Array` to account for the array offset.

### Are these changes tested?

Yes, I've added new unit tests.

### Are there any user-facing changes?

Yes, this is a user-facing bug fix.
* GitHub Issue: #41199

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