arrow

GH-38348: [C#] Make PrimitiveArray<T> support IReadOnlyList<T?> (#38680)

### What changes are included in this PR?

Make Arrow arrays of scalar type T implement the same semantic contract as IReadOnlyList<T?>.
Note that this PR does not include similar support for ICollection<T?>. I could add that support in this PR or a future PR.

### Are these changes tested?

This PR includes unit tests of the implemented IReadOnlyList<T?> methods.
* Closes: #38348

Authored-by: voidstar69 <voidstar69@gmail.com>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>