Unify codegen tool and build directory handling

[linmajia]

This PR cleans up rDSN build/codegen path handling across Windows and Unix-like platforms.

Changes

• Remove the stale bin/Linux/thrift ignore entry.
• Download missing Windows protoc.exe prebuilt tool.
• Install Windows prebuilt thrift.exe and protoc.exe into builder/output/bin/${CMAKE_SYSTEM_NAME}, matching the Unix codegen tool layout.
• Prefer built Windows protoc.exe when available, falling back to the downloaded prebuilt one.
• Add DSN_BUILD_DIR support so build/test/install/publish/codegen can use a custom build directory instead of hardcoded builder.
• Make Windows default build/test/install commands use builder, consistent with Unix.
• Update Windows install documentation accordingly.
• Avoid hardcoded top-level build paths in IDL codegen tests.