Quant tool: make removal of Clip/Relu ops configurable (#20616)
### Description
Adds the extra option `QDQKeepRemovableActivations` to optionally
prevent automatic removal of Clip/Relu ops in QDQ models. The current
default behavior, which is to remove Clip/Relu, remains the same if the
new option is not enabled.
### Motivation and Context
Explicitly representing these Relu/Clip operators in the QDQ model is
necessary if optimizations or EP transformations will later remove
QuantizeLinear/DequantizeLinear operators from the model.