build: plugins: Generate plugin defs on separated build directories

[cosmo0920]

When building in the several build directories, the generated flb_plugins.h would be collided.
So, we need to manage in the different directory for the generated header.

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Testing
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• Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

Summary by CodeRabbit

• Chores
  • Updated the build process to generate plugin headers in the build directory rather than modifying the source tree.
  • Improved build configuration to locate generated headers automatically.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 90765b68-c7c7-4994-8c68-91bf320ab44a

📥 Commits

Reviewing files that changed from the base of the PR and between 064e90b and 9cab86e.

📒 Files selected for processing (2)

• cmake/headers.cmake
• plugins/CMakeLists.txt

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📝 Walkthrough

Walkthrough

CMake now generates flb_plugins.h under the build directory and adds the generated include directory to the compiler include paths.

Changes

Generated Header Build Path

Layer / File(s)	Summary
Binary-tree generated header wiring plugins/CMakeLists.txt, cmake/headers.cmake	Creates the binary-tree output directory, writes flb_plugins.h there, and exposes ${FLB_PATH_ROOT_BINARY_DIR}/generated/include to builds.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: edsiper, celalettin1286

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title matches the main change: generating plugin definitions in separate build directories to avoid collisions.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch cosmo0920-use-separated-generated-plugin-defs

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