WebNN: Support sign operator for DirectML and tflite backends

This CL defines the sign operation in idl, mojom and implements it for
DirectML and tflite backends.

This CL also adds data type support limits for sign operation and adds
some related WPT validation and conformance tests.

Bug: 40206287, 345271830
Change-Id: I9425c3dfe0e7ac5bf45816cb0bf1ab881e9f0860
Cq-Include-Trybots: luci.chromium.try:win11-blink-rel, mac14.arm64-blink-rel, mac14-blink-rel, mac15.arm64-blink-rel, mac15-blink-rel, linux-blink-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5812182


Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Commit-Queue: Bin Miao <bin.miao@intel.com>
Reviewed-by: Weizhong Xia <weizhong@google.com>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1349514}