Extracting arrow-pg library

[sunng87]

One thing that always in my plan is to extract our encoding code from datafusion-postgres into a new crate called arrow-pg. This crate is responsible for data type mapping from Arrow to Postgres, and vice versa. It will take care of text and binray encoding/decoding for postgres. There will be a lot of corner cases to cover. Postgres client has a few options to control the encoding.

Eventually this library will unify all the arrow data encoding in:

• datafusion-postgres
• greptimedb
• some other pgwire + arrow scenario like the duckdb example in pgwire