Add in a vendor copy of Google Highway

[phongn]

Google Highway is a C++ library that provides portable SIMD/vector intrinsics. It currently supports all targets that ATS intends to build for (notably x86_64 and aarch64), and offers runtime dynamic dispatch.

Profiling of ATS indicates we have a number of string processing hotspots that are amenable to vector acceleration. However, each microarchitecture has different intrinsics and on x86_64 this varies wildly. Rather than take up the maintenance burden of maintaining and updating hand-written intrinsic code, we instead can use Highway to generate it for us.

This pull request adds a snapshot of Highway 1.4.0, but without tests, documentation or non-CMake build infrastructure.

[masaori335]

Do we really need to copy the lib in our repo? I thought let CMake to find the external library like what we do for jemalloc, hwloc, ...etc.

https://github.com/apache/trafficserver/blob/master/cmake/Findjemalloc.cmake
https://github.com/apache/trafficserver/blob/master/cmake/Findhwloc.cmake

[masaori335]

NVM, we agreed with vendor copy at weekly bug scrub.

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[masaori335]

Thank you!