OAuth refactor: install cdt_identity app

[thekaveman]

Python dependency

To make this refactor simpler, we aren't publishing a django-cdt-identity package to PyPI yet.

For now, we can just install directly from the main branch using the git URL syntax for dependencies:

[project]
name = "benefits"
...
dependencies = [
   ...
   "Django==5.1.7",
+  "django-cdt-identity @ git+https://github.com/Office-of-Digital-Services/django-cdt-identity.git@main",
   ...
]

Django dependency

Install the app in benefits/settings.py:

# Application definition

INSTALLED_APPS = [
    ...
+   "cdt_identity",
    "benefits.core",
    "benefits.enrollment",
    "benefits.eligibility",
    "benefits.oauth",
    "benefits.in_person",
]