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GH-23995: [C#] Make PrimitiveArrayBuilder constructor public (#44596)

Fixes #23995.

Making these constructors `public` allows for writing custom builders.

### Rationale for this change

Allows for writing custom builders.

### What changes are included in this PR?

Only change of visibility on the default constructors of `PrimitiveArrayBuilder`.

### Are these changes tested?

There is not much to test :)

### Are there any user-facing changes?

See above.

* GitHub Issue: #23995

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