feat: add X-BuilderNet-SentAtUs header handling

[mempirate]

Adds the option to extract the X-BuilderNet-SentAtUs header to the Go proxy. Allows 2 connected proxies to establish some latency stats on exchanged messages, which is useful in benchmarking.

[Copilot]

Pull Request Overview

Adds support for extracting the X-BuilderNet-SentAtUs header to enable latency statistics between connected proxies for benchmarking purposes.

• Introduces header extraction configuration and parsing logic for microsecond timestamps
• Adds context storage and accessor function for the parsed timestamp
• Refactors hardcoded header names to use constants for better maintainability

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[Copilot]

The header parsing silently ignores errors when the timestamp is invalid or missing. Consider logging invalid header values for debugging purposes, as this could help identify issues during benchmarking.

Suggested change

	}
	}
	} else {
	slog.Warn("Invalid or missing builderNetSentAtHeader value", "header", builderNetSentAtHeader, "value", builderNetSentAt, "error", err)
	}