docs: Add ADR for DiscussionsConfiguration HTTP API [BD-03]

openedx/core/djangoapps/discussions/docs/decisions/0003-configuration-rest-api.rst

+Expose Discussion Configuration via HTTP API
+============================================
+
+
+Status
+------
+
+Proposal
+
+
+Context
+-------
+
+As part of the BD-03 initiative (Blended Development, Project 3),
+we have previously created a discussion provider configuration backend,
+`DiscussionsConfiguration`, as well as a new microfrontend (MFE),
+`frontend-app-course-authoring`.
+
+However, these two systems cannot yet interact with one another.
+
+This document proposes the creation of a new HTTP API to connect the two.
+
+
+Requirements
+------------
+
+For a given `context_key`, this API must allow:
+- retrieval of:
+  - the list of available providers
+  - any options for the active provider
+- creation of:
+  - new configurations
+- updating of:
+  - existing configurations
+- deletion/disabling of:
+  - unneeded/inactive configurations
+
+
+Consideration
+-------------
+
+The API should follow existing best-practices and technology as exists
+in `edx-platform`. We should _not_ introduce new API architecture.
+
+
+Decision
+--------
+
+We propose to implement this as a Django REST Framework (DRF)-based HTTP API.
+
+This API will provide the following HTTP methods:
+- GET
+  - Retrieve collection of active and available providers,
+    as well as their options
+- POST
+  - Create, update, or disable a configuration
+
+
+Payload Shape
+-------------
+
+The payload is expected to be shaped like this (key names subject to change):
+
+.. code-block:: python
+    payload = {
+        'context_key': str(configuration.context_key),
+        'enabled': configuration.enabled,
+        'features': {
+            'discussion-page',
+            'embedded-course-sections',
+            'lti',
+            'wcag-2.1',
+        },
+        'plugin_configuration': configuration.plugin_configuration,
+        'providers': {
+            'active': configuration.provider_type or '',
+            'available': {
+                provider: {
+                    'features': PROVIDER_FEATURE_MAP.get(provider) or [],
+                }
+                for provider in configuration.available_providers
+            },
+        },
+    }
+
+The following configuration values are explicitly omitted;
+this should be left entirely up to the MFE.
+-  name
+-  logo
+-  description
+-  support_level
+-  documentation URL
+
+The LTI configuration (keys, secrets, URLs, etc.) are considered
+out-of-scope at this time.