chore: automatically detect openssl on macos

[Spockkk0225]

While building on macos, it will detect real OpenSSL path and set OPENSSL_ROOT_DIR to brpc.

# macOS: auto-detect Homebrew OpenSSL path (arm64 uses /opt/homebrew, x86_64 uses /usr/local)
if (APPLE AND NOT OPENSSL_ROOT_DIR)
    execute_process(
        COMMAND brew --prefix openssl@3
        OUTPUT_VARIABLE BREW_OPENSSL_PREFIX
        OUTPUT_STRIP_TRAILING_WHITESPACE
        ERROR_QUIET
    )
    if (BREW_OPENSSL_PREFIX AND EXISTS "${BREW_OPENSSL_PREFIX}")
        set(OPENSSL_ROOT_DIR "${BREW_OPENSSL_PREFIX}")
        message(STATUS "Using Homebrew OpenSSL: ${OPENSSL_ROOT_DIR}")
    endif()
endif()

fix #484

[zhanglei1949]

There seems to be multiple find_package(OpenSSL) in our CMakeLists, maybe we should replace all find_package(OpenSSL) with a new macro, which ensure OpenSSL in correctly found.
https://github.com/search?q=repo%3Aalibaba%2Fneug+find_package%28OpenSSL&type=code

[zhanglei1949]

LGTM

[zhanglei1949]

Maybe we should do find_package(OpenSSL) if OPENSSL_ROOT_DIR 1is not specified, then set the prefix to OPENSSL_ROOT_DIR. This should be done in BuildBrpcxxx.cmake

[zhanglei1949]

LGTM