Add support for django tenants integration

[onGranot]

Subject: Supporting django-tenants from django-redshift-backend

Problem

• We are using the django-tenants extension, which utilizes a schema-based tenant model. Currently queries will not set their search path to the tenant schema in the redshift backend.

Procedure to reproduce the problem

1. Initialize a project with https://django-tenants.readthedocs.io/en/latest/
2. Try to query django-redshift-backend on a specific tenant.

Expected results

An optional flag, when enabled the redshift backend will execute a set search path on the cursor.

Environment info

• OS: Mac (local), Ubuntu (deployment)
• Python version: 3.9.17
• Django version: 4.0
• Django-Redshift-Backend version: latest

[onGranot]

I have a working base.py optional override using a django setting that I could share, if you want to give me permissions to open a PR.

[shimizukawa]

@onGranot Thank you for your contribution! I think anyone can create PR, so please give it a try.
Actually, I have never given individual permission to create PR.