Reuse a Faraday client per Trino::Client instance

[p623]

Purpose

Reuse a single Faraday HTTP client for each Trino::Client instance.

Currently, query operations create a new Faraday client for each request.
Reusing the client reduces connection and TLS handshake overhead and enables HTTP keep-alive with a compatible Faraday adapter.

This PR is based on the idea and implementation proposed in #137. Since that PR has not been updated for over a year, this PR provides a fresh implementation for the current codebase with expanded test coverage.

Thank you to @redox for the original proposal and implementation.

Overview

• Reuse one Faraday client per Trino::Client instance
• Set query-specific HTTP headers for each request
• Preserve the existing behavior of query operations
• Add extensive tests for client reuse and request-specific behavior
• Update the implementation for the current codebase

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary