Cleanup > Reorganize 'external' code.

[argmarco-tkd]

Describe the enhancement requested

Current state

• DBPA-specific code is split between cpp/src/parquet/encryption/external/ (e.g., dbpa_executor.*) and top-level files like cpp/src/parquet/encryption/external_dbpa_encryption*.cc
• Generic “external encryptor” concerns and DBPA provider logic are not clearly separated.
• CMake targets don’t mirror the directory structure, making ownership and dependencies harder to track.

Desired end state

• Move most DBPA-specific sources from cpp/src/parquet/encryption/external/ into
  • cpp/src/parquet/encryption/external/dbpa/ (keep provider-agnostic utils at cpp/src/parquet/encryption/external/utils/*).
• Move cpp/src/parquet/encryption/external_dbpa_encryption.* into cpp/src/parquet/encryption/external/
• Restructure CMake to match the directories:
  • In encryption/CMakeLists.txt: add_subdirectory(external)
  • In encryption/external/CMakeLists.txt: define an external encryptor library/target; add_subdirectory(dbpa); link DBPA target against the external target; expose include paths matching the new structure; install headers to parquet/encryption/external/**.
  • In encryption/external/dbpa/CMakeLists.txt: define the DBPA provider target and its deps.

Component(s)

C#, C++

[avalerio-tkd]

Deprecated. Won't fix