Introduce LtiConfigView

So this is just an idea - feel free to shut this down if this doesn't
really fit into the vision of the django-lti library.

Basically, I had an idea to add a new `config.json` route which would
depend on the registration uuid. At least with Canvas LMS, this endpoint
can be used to autoconfigure the LTI application, making integration
much simpler at the admin level. This way the application author can
specify exactly where the LTI App intends to be placed within the LMS,
like the XML config schema we were using in LTI 1.1. This is much more
reliable than manually configuring these fields each time the LTI
application is installed.

I'm assuming this will require some discussion and refinements, so I'm
opening this up as a draft PR.

This is something the application author can create themselves, here
is an example of that:
* https://github.com/ccnmtl/mediathread/blob/master/lti_auth/views.py#L135

And I've generalized that view which is included here.

I think that anything django-lti can pre-configure itself like this
would be beneficial to application developers. Personally I've found
LTI 1.3 pretty complicated to work with as it is, so I'd like for
a library like this to simplify whatever it can.

README.md

@@ -48,11 +48,12 @@ To allow LTI platforms to retrieve a JWKS and initiate a launch, add paths for
 ```python
 ...
 
-from lti_tool.views import jwks, OIDCLoginInitView
+from lti_tool.views import jwks, OIDCLoginInitView, LtiConfigView
 
 urlpatterns = [
     path(".well-known/jwks.json", jwks, name="jwks"),
     path("init/<uuid:registration_uuid>/", OIDCLoginInitView.as_view(), name="init"),
+    path("<uuid:registration_uuid>/config.json", LtiConfigView.as_view()),
 ]
 
 ```

lti_tool/views.py

@@ -1,3 +1,4 @@
+from django.conf import settings
 from django.http import (
     HttpRequest,
     HttpResponse,
@@ -6,10 +7,12 @@
     JsonResponse,
 )
 from django.http.response import HttpResponseRedirect
+from django.urls import reverse
 from django.utils.decorators import method_decorator
 from django.utils.translation import gettext as _
 from django.views import View
 from django.views.decorators.csrf import csrf_exempt
+from urllib.parse import urljoin
 
 from pylti1p3.contrib.django import DjangoCacheDataStorage, DjangoOIDCLogin
 
@@ -140,3 +143,55 @@ def handle_data_privacy_launch(
         if return_url is None:
             return HttpResponseForbidden(error_msg)
         return HttpResponseRedirect(return_url)
+
+
+class LtiConfigView(View):
+    """
+    JSON configuration endpoint for LTI 1.3.
+
+    In Canvas LMS, an LTI Developer Key can be created via Manual
+    Entry, or by URL. This view provides the JSON necessary for URL
+    configuration in Canvas.
+
+    https://canvas.instructure.com/doc/api/file.lti_dev_key_config.html
+    """
+    @staticmethod
+    def get_config_json(domain: str, uuid: str) -> dict:
+        oidc_init_uri = urljoin(
+            "https://{}".format(domain),
+            reverse("init", kwargs={"registration_uuid": uuid}))
+
+        return {
+            "title": settings.LTI_TOOL_CONFIGURATION["title"],
+            "description": settings.LTI_TOOL_CONFIGURATION["description"],
+            "oidc_initiation_url": oidc_init_uri,
+            "scopes": [
+                "https://purl.imsglobal.org/spec/lti-ags/scope/lineitem",
+                "https://purl.imsglobal.org/spec/lti-ags/scope/result.readonly"
+            ],
+            "public_jwk_url": urljoin(
+                "https://{}".format(domain), reverse("jwks")),
+        }
+
+    def config_hook(self, config: dict) -> dict:
+        """
+        Override this method at the application level to customize the
+        configuration object. At the very least, `target_link_uri`
+        must be added to the config dictionary.
+
+        You can configure placements for the LTI application within
+        the LMS tool via options in the returned dictionary.
+
+        See reference documentation:
+        https://canvas.instructure.com/doc/api/file.lti_dev_key_config.html
+        """
+        return config
+
+    def get(self, request: HttpRequest, *args, **kwargs) -> JsonResponse:
+        domain = request.get_host()
+        registration_uuid = kwargs.get("registration_uuid")
+        config_obj = LtiConfigView.get_config_json(domain, registration_uuid)
+
+        config_obj = self.config_hook(config_obj)
+
+        return JsonResponse(config_obj)