build(cmake): Modernize target-scoped CMake configuration

[darion-yaphet]

What problem does this PR solve?

Issue Number: N/A

Problem Summary:
The CMake build files still rely on legacy global include directories, compile flags, and duplicated per-example configuration. This makes target behavior harder to
reason about and causes standalone examples/tests to drift from the main build configuration.

What is changed and the side effects?

Changed:

• Modernize top-level, src, tools, and test CMake targets with target-scoped configuration.
• Add shared brpc_common_config for common compile definitions, include paths, options, and features.
• Refactor standalone example CMake files to use example/cmake/BrpcExample.cmake.
• Update the gtest download helper to use CMake 3.16...3.28.
• Ensure downloaded gtest headers take priority over Homebrew's C++17-only gtest headers during unit-test builds.

Side effects:

• Standalone examples now consistently require CMake 3.16 or newer, matching the main project requirement.

• DOWNLOAD_GTEST=ON creates a copied gtest include directory in the test build tree to avoid header conflicts with system-installed gtest.

• Performance effects:
  No runtime performance impact expected. This is a build-system-only change.

• Breaking backward compatibility:
  No source/API compatibility break expected. Users building standalone examples with CMake older than 3.16 will need to upgrade CMake.

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

Verification

Passed:

• cmake -S . -B build -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl@3
• cmake --build build -j6
• cmake -S . -B build-tests -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl@3 -DBUILD_UNIT_TESTS=ON -DDOWNLOAD_GTEST=ON
• cmake --build build-tests -j6
• Standalone example smoke builds for echo_c++, http_c++, and memcache_c++

Known macOS arm64 test issues:

• test_butil fails in StackTraceTest.find_symbol
• bthread_fd_unittest fails in FDTest.ping_pong
• rdma_performance standalone example was not tested because ibverbs is unavailable on macOS

[chenBright]

The upgraded version seems too high, is it necessary?

[darion-yaphet]

The upgraded version seems too high, is it necessary?

I understand the concern about compatibility, but CMake 3.16 is actually not very radical. CMake 2.x was quite an old era. The main line has moved to 4.x now, and CMake 3 is the mainstream baseline for many years past.

The main benefits of upgrading to CMake 3 are not "keeping up with the latest trends", but rather the ability to use modern CMake syntax: target-based configuration, target_compile_features, target_link_options, generator expressions, better dependency discovery and cross-platform behavior. These features can avoid the pollution of global CFLAGS/LDFLAGS, allowing includes, macro definitions, and link parameters to be propagated by target. This makes the project easier to maintain and less likely to fail due to differences in the local environment.

Overall, 3.16 represents a relatively conservative modern baseline, not a radical upgrade.