[QDQ Quantization] Refactor shared functionality into a base quantizer (#19817)

### Description
This PR does not add or remove any functionality. It refactors common
functionality shared by the `ONNXQuantizer` and `QDQQuantizer` classes
into a new `BaseQuantizer` class.

This change helps decouple the QDQ quantizer from other quantization
modes and makes it easier to determine if a change to one quantization
mode will impact another.

### Motivation and Context
An upcoming PR aims to add mixed-precision support to QDQ models (e.g.,
one part of the graph uses u8 activations and another uses u16
activations). This change makes the upcoming PR smaller and should
presumably make determining the impact on existing features more
straightforward.

Author: adrianlizarraga