Refactor output plugin name and packages

[nickytd]

How to categorize this PR?
/kind cleanup
/area logging

What this PR does / why we need it:
This PR refactors the Fluent Bit output plugin codebase by renaming packages and files for better consistency and clarity. The main changes include renaming the pkg/valiplugin package to pkg/plugin, updating the command directory from cmd/fluent-bit-vali-plugin to cmd/fluent-bit-output-plugin, and standardizing interface names from Vali to OutputPlugin. This refactoring improves code organization and makes the plugin structure more generic and maintainable.

Code changes:

• Renamed pkg/valiplugin package to pkg/plugin and updated all references
• Renamed command directory from cmd/fluent-bit-vali-plugin to cmd/fluent-bit-output-plugin
• Updated interface name from Vali to OutputPlugin throughout the codebase
• Modified build targets in Makefile to use new naming conventions (e.g., out_vali.so → output_plugin.so)
• Updated Dockerfile to reflect new plugin binary names and paths
• Refactored test directories from tests/valiplugin to tests/plugin
• Updated copyright headers and REUSE configuration for renamed files
• Removed unused NewValiClientDecoratorFunc type from client types

Additional context:
This is a structural refactoring that maintains the same functionality while improving code organization. The changes make the plugin more generic by removing Vali-specific naming conventions, which aligns with the goal of creating a more flexible output plugin architecture.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

Refactored Fluent Bit output plugin package structure by renaming pkg/valiplugin to pkg/plugin and updating related directories and interfaces for better code organization

• 🔄 Regenerate Summary

[gardener-robot]

@nickytd Label kind/todo does not exist.

[gardener-robot]

@hyperspace-insights[bot] Command /add is not available to you but only to a Maintainer, Member, Author.

[gardener-robot]

@hyperspace-insights[bot] Command /add is not available to you but only to a Maintainer, Member, Author.

[gardener-robot]

@hyperspace-insights[bot] You have mentioned internal references in the public. Please check.

[gardener-robot]

@nickytd You have mentioned internal references in the public. Please check.

[gardener-robot]

@hyperspace-insights[bot] Command /add is not available to you but only to a Maintainer, Member, Author.

[gardener-robot]

@hyperspace-insights[bot] Command /add is not available to you but only to a Maintainer, Member, Author.

[gardener-robot]

@rrhubenov, @dnaeon You have pull request review open invite, please check

[rrhubenov]

/lgtm