[C#] Change the class access modifier of PrimitiveArrayBuilder to public

[asfimport]

Currently, a user seeking to add a Builder<T> class to their user-defined type in C# can subclass PrimitiveArrayBuilder since it is marked public abstract. However, it's parameterless constructor is marked as internal making it impossible to compile the user's class.

The preferred solution would be to mark this constructor as public. If this cannot be done for some reason, the entire </font><font color="#172b4d">PrimitiveArrayBuilder</font><font color="#172b4d"> class should be marked internal to avoid confusion.

Reporter: Zachary Gramana

_{Note: This issue was originally created as ARROW-7757. Please see the migration documentation for further details.}

[CurtHagenlocher]

Issue resolved by pull request 44596
#44596