Update int/float size for PostgreSQL, add PostgreSQL tests and fix various bugs

[EricFecteau]

Goal

The goal of this pull request is to:

• Make the size of the int and float smaller (e.g. don't convert all i16 to i64) -- this adds the framework for Int dtypes are unnecessarily big #688 for Postgres -> Polars, but not mysql -> Polars)
• Add tests for these types (and all other types) - mostly in Postgres for now

Comments

• While writing more tests, I changed the PostgreSQL examples, so had to update the relevant tests in Rust and Python.
• I wrote tests for the the 4 protocols for PostgreSQL: Binary, CSV, Cursor and Simple
• Made clippy happy in various places (mostly PostgreSQL)

Bugs

While writing tests, I encountered a few bugs and solved them:

• Bug when Null are included inside an array (all types) #736 Null within an array would fail in PostgreSQL! I implemented this to work in Rust. I did not implement this for Pandas (but kept it as-is, with an unimplemented!
• Bug in TimeStamp with Time Zones for Arrow #735 The UTC time being used in the crate was not recognized by Arrow, due to the lack of the chrono-tz feature. Fixed by moving to +00:00 instead.
• The PostgreSQL CSV and Simple protocol did not implement millisecond for time, so I implemented it.
• Bool was incorrectly implemented for the PostgreSQL Simple protocol, so I fix it!

[wangxiaoying]

Hi @EricFecteau , thanks for the PR! It looks good. I only left a few comments.