Add retry to POST /v1/statement

[murakami-ta]

Purpose

In our environment, POST /v1/statement occasionally fails with HTTP errors, so i want the trino-client-ruby gem to retry.
GET requests already retry Faraday errors inside the gem via faraday_get_with_retry, so i want POST to be retried inside the gem in the same way.

Overview

• Extract the retry logic from faraday_get_with_retry into a shared with_retry_loop helper used by both GET and POST
• Route post_query_request! through with_retry_loop to retry on Faraday::TimeoutError, Faraday::ConnectionFailed, and 502/503/504
• Add specs for POST retry

Checklist

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