ENH: add schema support to sql functions

Related #7441. Work in progress for now

- [x] support via a `schema` arg in the `read_sql_table` and `to_sql` functions
- [ ] not for `read_sql_query` -> is this needed? (this is a direct `execute`, and I don't directly see how this can be aware of schema's)
- [x] tested for postgresql
- [ ] test for other databases (mysql)
- [ ] docs