Add missing functions for Redshift

[Smlep]

Describe the bug
Many redshift functions cannot be mocked, and should be implemented as UDFs

To Reproduce

from pytest_mock_resources import create_redshift_fixture
from sqlalchemy.orm import Session
from sqlalchemy.sql import text

session = create_redshift_fixture(session=True)


def test_len(session: Session):
    q = f"select len('apple')"

    res = session.execute(text(q))
    item = res.fetchone()

    assert item is not None
    assert item[0] == 5

Expected behavior
Test success

Actual Behavior

E       sqlalchemy.exc.ProgrammingError: (psycopg2.errors.UndefinedFunction) function len(character varying) does not exist
E       LINE 1: select len('apple')
E                      ^
E       HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
E
E       [SQL: select len('apple')]
E       (Background on this error at: https://sqlalche.me/e/20/f405)

Additional context
This happens because the LEN function is not handled by sqlalchemy as is, and is not defined in the custom fixtures

A non-exhaustive list of functions I frequently use that are missing:

• Len
• Median
• Convert_timezone
• Listagg

I have a PR ready for LEN to begin with, I will try to implement the other ones later.

I can split this into multiple issues if you prefer

[Smlep]

Median and listagg might not be as easy to use since Redshift does not allow UDFs for aggregate operations