Add Presto Go Client v2 RFC

[ethanyzhang]

Summary

Proposes adopting presto-go as the official Presto Go client v2, replacing the unmaintained prestodb/presto-go-client.

The v2 client is already in production use by pbench and addresses longstanding gaps in the v1 client:

• Modular auth — Kerberos and OAuth2 (with token refresh) as opt-in separate Go modules, eliminating the forced gokrb5 dependency
• Query introspection APIs — /v1/query, /v1/cluster, /v1/queryState for monitoring and admin tooling
• query_json subpackage — structured types for parsing query info/stats, with derived metrics for performance analysis
• Go generics — NullSlice[T], NullMap[K,V], NullRow[T] replace ~30 explicit scanner types
• Trino compatibility — automatic header translation for cross-engine benchmarking
• Mutual TLS, OAuth2 client credentials flow, all 4 Presto isolation levels
• Thread-safe sessions, batch streaming with Drain, retry body buffering
• prestotest mock server — stdlib-only, reusable by downstream apps for hermetic testing
• CI — 80% coverage threshold, lint, govulncheck

No breaking changes to v1. Users opt in via a new module path with a documented migration guide.