WebNN: Blink side changes to implement the cast operator

This change completes
https://chromium-review.googlesource.com/c/chromium/src/+/5050358
and adds the blink side changes to invoke cast operator.

One thing to note is that on the blink side the cast operator starts out
not quite as an unary operator, because its idl definition needs the
data type specification for the output operand.

MLGraphBuilder::cast creates an output operand with that data type
information and after that point cast is similar to every other
unary operator.

Bug: 1273291
Change-Id: Ie3699489eeeefe26dab5a0b2245347e35ff15516
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5056249
Reviewed-by: Jiewei Qian <qjw@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: ningxin hu <ningxin.hu@intel.com>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Commit-Queue: Sushanth Rajasankar <Sushraja@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1230115}