Bring back proxy configuration for vali client"

[nickytd]

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

What this PR does / why we need it:
This PR restores proxy configuration support for the Vali client in the fluent-bit output plugin. The changes enable users to configure HTTP proxy settings for the Vali client through the ProxyUrl configuration parameter, allowing the plugin to work in environments where direct internet access is restricted and proxy servers are required.

Code changes:

• Added ProxyUrl to the list of extractable configuration keys in the fluent-bit output plugin
• Implemented processProxyURLConfig function to parse and validate proxy URL configuration
• Added proxy URL processing to the configuration post-processing pipeline
• Enhanced configuration dumping to include proxy URL information for debugging
• Updated test cases to verify proxy URL configuration parsing and validation

Additional context:
The proxy configuration follows the same pattern as the main URL configuration, supporting both ProxyURL and ProxyUrl parameter variants for flexibility. The implementation includes proper URL validation and error handling to ensure robust configuration parsing.

Which issue(s) this PR fixes:

Special notes for your reviewer:
This change maintains backward compatibility as the proxy configuration is optional. The implementation reuses standard Go net/url parsing for consistency and reliability.

Release note:

Added proxy configuration support for Vali client through ProxyUrl parameter

• 🔄 Regenerate and Update Summary

[gardener-robot]

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[gardener-robot]

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[nickytd]

@MichaelEischer, here it is.

[MichaelEischer]

Thanks!