[Enhancement] Migrate Paimon C++ dependency to official Apache release v0.3.0

[hulincup]

Problem fixed & how

Paimon C++ has been migrated to the Apache community (https://github.com/apache/paimon-cpp), but Doris was still using an older snapshot from apache/doris-thirdparty (commit 0a4f4e2). This PR migrates the dependency to the official Apache Paimon C++ v0.3.0 release.

Changes:

• Updated thirdparty/vars.sh to download from apache/paimon-cpp v0.3.0 instead of doris-thirdparty snapshot
• Created new patch paimon-cpp-v0.3.0-external-arrow.patch to support external Arrow integration (aligning with paimon-cpp-cache.cmake which already configures PAIMON_USE_EXTERNAL_ARROW)
• Updated thirdparty/download-thirdparty.sh to apply the new patch

Behavior modified

Previous: Doris downloaded Paimon C++ from apache/doris-thirdparty snapshot (commit 0a4f4e2), which is a stale copy without version tracking.

Now: Doris downloads Paimon C++ from the official Apache release apache/paimon-cpp v0.3.0, making it easier to receive future updates and bug fixes.

Impact: No functional changes to Doris users. This is a dependency migration that improves maintainability and alignment with the Apache Paimon community.

Features added

N/A - This is a dependency migration, not a new feature.

Code refactored

N/A - No refactoring required.

Functions optimized

N/A - No performance optimizations.

Test plan

• Verify thirdparty build succeeds with the new download URL
• Verify the patch applies cleanly to paimon-cpp v0.3.0
• Verify BE build succeeds
• Verify Paimon-related tests pass

Note: Full build verification requires a complete Doris build environment. This PR focuses on the dependency migration; CI will validate the build.

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[hulincup]

1. What problem was fixed

Migrated Paimon C++ dependency from the old snapshot in apache/doris-thirdparty (commit 0a4f4e2) to the official Apache release v0.3.0 at apache/paimon-cpp.

The old snapshot is stale and doesn't receive updates. The official Apache repository is the canonical source for Paimon C++ releases.

2. Which behaviors were modified

Previous behavior: Doris downloaded Paimon C++ from apache/doris-thirdparty snapshot, which is a community-maintained mirror that may lag behind official releases.

New behavior: Doris now downloads from the official Apache Paimon C++ v0.3.0 release at apache/paimon-cpp.

Why: To align with the Apache Paimon community's official release process and make it easier to receive future updates and bug fixes.

Impact: No functional changes for Doris users. This is a dependency source migration only.

3. What features were added

None. This is a dependency migration, not a feature addition.

4. What code was refactored

None. No refactoring was needed.

5. What functions were optimized

None. No performance optimizations.

Changes made

1. thirdparty/vars.sh: Updated download URL from doris-thirdparty snapshot to apache/paimon-cpp v0.3.0 release
2. thirdparty/patches/paimon-cpp-v0.3.0-external-arrow.patch: New patch to support external Arrow integration (required because v0.3.0 has different CMake structure than the old snapshot)
3. thirdparty/download-thirdparty.sh: Updated to apply the new patch file