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]

LGTM