Support PostgreSQL 'point' type

[SindriFr]

Support is missing for postgres geometric types, namely 'point'.

Motivation

Postgres has built-in support for geometric types but they cannot be used with sea-query models. I've tried to use the newtype pattern to work around this, but Value does not contain any appropriate variant which results in a type error from the database.

Proposed Solutions

Add Value::Point variant containing sqlx::postgres::types::PgPoint which implements the necessary traits. I've drafted PRs with the necessary changes for sea-query and sea-orm, but I welcome any feedback.