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GH-38483: [C#] Add support for more decimal conversions (#38508)

### What changes are included in this PR?

Adds support for decimal128 <-> string and decimal256 <-> string.
Adds support for decimal256 <-> SqlDecimal.

### Are these changes tested?

Yes

### Are there any user-facing changes?

APIs have been added to support these conversions.
* Closes: #38483

Authored-by: Curt Hagenlocher <curt@hagenlocher.org>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>