Description of changes:

Some platform are removing the openssl/engine.h header, which causes s2n-tls builds to fail (#4705, #4873).

This PR splits the static S2N_LIBCRYPTO_SUPPORTS_CUSTOM_RAND check into a:

• runtime check if s2n-tls custom random is supported
• feature-probe if the linked libcrypto supports ENGINE apis

Additional benefits:
The split limits the scope of the conditional compilation to ENGINE related features. The feature probe is also more comprehensive and flexible than the static check (eg. check for the openssl/engine.h header).

Existing checks (S2N_LIBCRYPTO_SUPPORTS_CUSTOM_RAND):

• is_openssl: (runtime check)
• not fips: (runtime check)

New checks:

• Check if ENGINE related APIs are defined: (feature probe)
• Check RAND_METHOD signature: (feature probe. due to awslc signature differences)

Testing:

I added a negative test for the feature probe. The positive test is missing due to unrelated feature probe failure on AL2. There is an issue for AL2 and I verified that only AL2 was failing by testing this commit in isolation. GeneralBatch Fail vs Pass after the test was removed.

Manual testing
I build s2n-tls linked to an openssl configured and build with the no-engine option.

### build openssl
git clone git@github.com:openssl/openssl.git openssl_no_engine

pushd openssl_no_engine;
    ./Configure no-engine --prefix=/home/toidiu/projects/s2n-tls/local_toidiu_dir_libcrypto  /openssl_no_engine/install
    # can be used to check that `OPENSSL_NO_ENGINE` is disabled
    # ./configdata.pm --dump
    make -j 16
    make install
popd

# Build s2n-tls run unit tests.
 
CMAKE_PREFIX_PATH="local_toidiu_dir_libcrypto/openssl_no_engine/install" \
    cmake . -Bbuild -GNinja \
    -DBUILD_SHARED_LIBS=ON \
    -DCMAKE_BUILD_TYPE=Debug \
    -DCMAKE_EXPORT_COMPILE_COMMANDS=1 \
    -DUNSAFE_TREAT_WARNINGS_AS_ERRORS=ON \
    -DS2N_STACKTRACE=1

cmake --build ./build -j $(nproc)

CTEST_OUTPUT_ON_FAILURE=1 CTEST_PARALLEL_LEVEL=$(nproc) ninja -C build test

-------------------- Build output
...
-- feature S2N_LIBCRYPTO_SUPPORTS_ENGINE: FALSE

-------------------- All tests PASSED
...
100% tests passed, 0 tests failed out of 273

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