Add multi-client support

[codemonium]

TL;DR

Add support for using multiple Langfuse::Client instances while preserving the existing singleton client behavior.

Why

Some applications need to report to more than one Langfuse project from the same process, such as separate "Meeting Transcription" and "Note Summarization" projects. The SDK previously assumed singleton usage in a few internal paths, which made multi-client usage require downstream monkey patches.

Checklist

• Has label
• Has linked issue
• Tests added for new behavior
• Docs updated (if user-facing)

[codemonium]

There are still several usages of Langfuse.configuration.logger throughout the codebase (chat_prompt_client.rb, otel_attributes.rb, propagation.rb, and masking.rb).

I left them alone because the change already does what I want, and I don't know the best approach forward for logging.

Should logging be client-specific, or should there be one global logger?