Add PGO support

[zamazan4ik]

It would be interesting to see if Profile-Guided Optimization (PGO) could help achieve better performance characteristics in PgCat.

A good starting point could be:

• Rust docs about PGO
• Kobzol blog
• cargo-pgo
• Repo about different PGO applications

Honestly not sure, if it could bring more QPS but probably can reduce CPU-overhead on PgCat side.