[WebNN] Support MatMulNBits op (#24142)
MatMulNBits op can be simply emulated by DequantizeLinear + Transpose +
MatMul and currently only 4-bit quantization is supported.
Thus the B and zero_points (if present) inputs must be known as
initializers with data type 'uint8' and we need to register them as
'uint4' WebNN constant.
Typically, all initializers are registered as WebNN constants in one
step via `ModelBuilder::RegisterInitializers` before constructing the
WebNN graph. However, due to WebNN doesn't support cast to 'uint4', we
need to defer the registration of these two inputs until the
`MatMulNBitsBuilder::AddToModelBuilderImpl` is invoked.
