out_loki: prevent race conditions when multiple workers use remove_keys

[edsiper]

Fixes #10560

Added per-thread removal contexts in Loki’s configuration to prevent race conditions when multiple workers use the Remove_Keys option. The change introduces a thread-local storage for these contexts and created helper functions to manage them safely

---

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.

[railsharipov]

@edsiper Thank you for the work you've done.
I am not sure if this change is the case but it is the only thing that I see that can be related: some keys not getting removed from Loki streams after I have updated to version 4.0.4. It is strange that some keys are removed while others aren't. This is just a guess so let me know if you think this change cannot be the cause.