[CPU] Add 8bit support to matmulnbits quantizer (#24384)
### Description
Add 8bit support to matmulnbits quantizer. matmul_4bits_quantizer now
can quantize a const B in a MatMul to 8bits initializer.
### Motivation and Context
MatMul4Bits has accuracy issue for phi-4 model used for foundry local.
The early prototype showed >= 6bits can fix the issue.
To mitigate the issue as soon as possible, add 8bit support to
MatMulNBits.
