[C++] AppendScalar implementation uses polymorphism

### Describe the enhancement requested

The `ArrayBuilder::AppendScalar` implementation is currently very odd:

* the `ArrayBuilder` hierarchy uses polymorphism (i.e. virtual methods) to handle type-specific operations
* but `ArrayBuilder::AppendScalar` is implemented entirely in the base class using a visitor, putting type-specific knowledge in the base class definition
* but `ArrayBuilder::AppendScalar` is *also* a virtual method overriden in dictionary and run-end-encoded builders
* furthermore, the visitor in the base class definition of `ArrayBuilder::AppendScalar` dispatched on the scalar's data type, instead of the builder's data type

We should refactor the `AppendScalar` implementation to be entirely polymorphic and remove any trace of type-specific implementations in the base class (`ArrayBuilder::AppendScalar` would probably become a pure virtual method).

This would also help in cases like https://github.com/apache/arrow/pull/50584

### Component(s)

C++

Start at ArrayBuilder::AppendScalar and trace the ArrayBuilder hierarchy, including the dictionary and run-end-encoded overrides mentioned in the issue. Confirm where scalar-type dispatch currently lives and identify the builder-specific entry points. Done means the base implementation contains no type-specific knowledge and each builder handles its own AppendScalar behavior polymorphically.

Written by the indexing model from the issue text.