Lib: Add new APIs to support creating processor unit for input and output

[yunzvanessa]

Summary

1. Add two new library APIs to support creating processor units for input and output

• flb_input_processor_unit(flb_ctx_t *ctx, const char *event_type, const char *processor_unit_name, int ffd, ...): create a processor unit with specific event type and processor unit name for an input plugin.
• flb_output_processor_unit(flb_ctx_t *ctx, const char *event_type, const char *processor_unit_name, int ffd, ...): create a processor unit with specific event type and processor unit name for an output plugin.

2. This PR also has a fix for processor which updates function signature of flb_processor_unit_create to use const char* for unit_name

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Enter [N/A] in the box, if an item is not applicable to your change.

Testing
Before we can approve your change; please submit the following in a comment:

• [N/A] Example configuration file for the change:

• Debug log output from testing the change:

# Example output log from an application that uses fluentbit as a C libary API. With the new processor unit APIs I added, I 
# enabled otel_envelop and content_modifier processor units to manipulate the log messages from the tail input plugin, 
# before sending the logs to openetelemetry output plugin

[2025/02/18 11:22:16] [debug] [input:tail:tail.0] inode=8603124, ErrorLog.txt, events: IN_ATTRIB
[2025/02/18 11:22:16] [debug] [input:tail:tail.0] inode=8603164, 8000_ErrorLog.txt, events: IN_ATTRIB
[2025/02/18 11:22:16] [debug] [input:tail:tail.0] inode=8603167, 8001_ErrorLog.txt, events: IN_ATTRIB
[2025/02/18 11:22:16] [debug] [input:tail:tail.0] inode=8594915, 8002_ErrorLog.txt, events: IN_ATTRIB
[2025/02/18 11:22:16] [debug] [input:tail:tail.0] inode=8594955, 7997_ErrorLog.txt, events: IN_ATTRIB
[2025/02/18 11:22:17] [debug] [input:tail:tail.0] inode=8603124, ErrorLog.txt, events: IN_MODIFY
[2025/02/18 11:22:17] [debug] [task] created task=0x7f22ec1853f0 id=1 OK
[2025/02/18 11:22:17] [debug] [output:stdout:stdout.1] task_id=1 assigned to thread #0
[2025/02/18 11:22:17] [debug] [out flush] cb_destroy coro_id=2
[2025/02/18 11:22:17] [ warn] [output:opentelemetry:opentelemetry.0] error performing HTTP request, remote host=remote.host.name:4318 connection error
[2025/02/18 11:22:17] [debug] [upstream] KA connection #65 to remote.host.name:4318 is now available
[2025/02/18 11:22:17] [debug] [upstream] KA connection #65 to remote.host.name:4318 has been disconnected by the remote service
[2025/02/18 11:22:17] [debug] [out flush] cb_destroy coro_id=1
[2025/02/18 11:22:17] [debug] [retry] new retry created for task_id=0 attempts=1
[2025/02/18 11:22:17] [ warn] [engine] failed to flush chunk '30926-1739906536.401558167.flb', retry in 8 seconds: task_id=0, input=tail.0 > output=opentelemetry.0 (out_id=0)
[2025/02/18 11:22:17] [debug] [input:tail:tail.0] inode=8603124, ErrorLog.txt, events: IN_MODIFY
[2025/02/18 11:22:17] [debug] [input:tail:tail.0] inode=8603124, ErrorLog.txt, events: IN_MODIFY
[2025/02/18 11:22:17] [debug] [input:tail:tail.0] inode=8603124, ErrorLog.txt, events: IN_MODIFY

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

# Output of running valgrind --leak-check=full --show-leak-kinds=all --track-origins=yes bin/fluent-bit
==27800== LEAK SUMMARY:
==27800==    definitely lost: 0 bytes in 0 blocks
==27800==    indirectly lost: 0 bytes in 0 blocks
==27800==      possibly lost: 0 bytes in 0 blocks
==27800==    still reachable: 99,872 bytes in 3,382 blocks
==27800==         suppressed: 0 bytes in 0 blocks
==27800==
==27800== For counts of detected and suppressed errors, rerun with: -v
==27800== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

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

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

Documentation

• [N/A] Documentation required for this feature

Backporting

• [N/A] 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.