WebNN: Define XNNPACK Node for pad

This CL implements DefineXnnNodeForPad() method that defines XNNPACK
pad Node by xnn_define_static_constant_pad(). This CL only supports
MLPadOptions->mode() = kConstant.

Besides, this CL also defines a new class MLPadOperator which inherits
MLOperator class. MLPadOperator class adds two member variables -
beginning_padding and ending_padding.

For unit tests, this CL implements PadTest of MLGraphTest that
checks the compute results of pad operators with constant mode.

Bug: 1273291
Change-Id: I79286ddea77801eafaf8f9350b8555fb621c1099
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4384074
Reviewed-by: Jiewei Qian <qjw@chromium.org>
Quick-Run: Alan Kelly <alankelly@google.com>
Commit-Queue: Lisha Guo <lisha.guo@intel.com>
Cr-Commit-Position: refs/heads/main@{#1127118}