build(example): extract shared CMake dependency helper for examples

[darion-yaphet]

What problem does this PR solve?

Problem Summary:

Example CMakeLists.txt files duplicate the same dependency discovery and link setup (brpc, protobuf, gflags, leveldb, openssl, thrift, Darwin flags). That makes
MySQL and other examples hard to maintain and easy to drift.

What is changed and the side effects?

Changed:

• Add brpc_example_find_common_deps in example/cmake/BrpcExample.cmake to centralize common dependency lookup and DYNAMIC_LIB construction.
• Migrate MySQL and other example CMakeLists.txt files to call the helper, removing duplicated find/link boilerplate.
• Implement the helper as a CMake macro so variables such as CMAKE_PREFIX_PATH, include paths, and library lists propagate to the caller scope.
• Keep example-specific deps local (e.g. MySQL readline/ncurses/thriftnb, redis/http gperftools).
  Side effects:
• Performance effects: None expected (build-system only).
• Breaking backward compatibility: Low risk. Example CMake entry points are simplified; users who build examples via each directory's CMakeLists.txt should use
  brpc_example_find_common_deps / existing BrpcExample.cmake helpers instead of the removed duplicated blocks. LINK_SO behavior is preserved.

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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.
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