Fix additional warnings found by the Undefined Behavior Sanitizer

[bjosv]

Fixes runtime error warnings found when running tests in the aws-crt-cpp repo with the undefined behavior sanitizer enabled.

cd aws-crt-cpp
mkdir build && cd build
CC=clang-18 CXX=clang++-18 cmake -G Ninja -DENABLE_SANITIZERS=ON -DSANITIZERS="undefined" ..
ninja
ninja test
grep "runtime error" Testing/Temporary/LastTest.log
...
aws-crt-cpp/crt/aws-c-common/source/hash_table.c:68:12: runtime error: call to function aws_byte_cursor_eq through pointer to incorrect function type 'bool (*)(const void *, const void *)'
...

Description of changes:
Add callback functions that uses correct signature aws_hash_callback_eq_fn (as also seen in aws-c-mqtt).

Since the CI-build is not using -fno-sanitize-recover=all, and the warnings are recoverable, they will not fail the CI build.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[bjosv]

As a update;
these warnings can also be seen when running the tests in aws-c-http itself.
There are 651 warnings in the 709 tests.

INSTALL_PATH=/tmp/aws-install   %% installed dependencies
cd aws-c-http
mkdir build && cd build
CC=clang-18 CXX=clang++-18 cmake -G Ninja -DCMAKE_PREFIX_PATH=${INSTALL_PATH} -DCMAKE_BUILD_TYPE=Debug -DENABLE_SANITIZERS=ON -DSANITIZERS="undefined" ..
ninja
ninja test
grep "runtime error: call to function aws_byte_cursor_eq" Testing/Temporary/LastTest.log | wc -l

651