feat(providers): add connection_config, credentials definition (#2768)

## Describe your changes

Fixes
https://linear.app/nango/issue/NAN-1792/connection-config-document-in-providersyaml-and-enforce-them

- Add and enforce `connection_config` definitions
This makes sure we have a JSON schema to validate and to display
connection config in the UI and backend.
The values are returned by the endpoint but not yet used in the UI. Some
things are incomplete or might be slightly incorrect, it's a daunting
task so I have fully checked and vested all of them.


- Add and enforce the `credentials` definitions for API_KEY
Same as `connection_config`, separated to clarify the expectation. 


## Tests
![Screenshot 2024-09-25 at 18 10
18](https://github.com/user-attachments/assets/730f23e0-a63b-404e-a2da-4f181c67624c)

.github/workflows/validation.yaml

@@ -28,6 +28,6 @@ jobs:
 
             - name: Validate YAML
               run: |
-                  cd scripts/validation/providers
-                  npm i
-                  node validate.js
+                  npm ci
+
+                  npx tsx scripts/validation/providers/validate.ts

docs-v2/spec.yaml

@@ -1712,6 +1712,24 @@ components:
       additionalProperties: false
       type: object
 
+    SimplifiedJSONSchema:
+        type: object
+        additionalProperties: false
+        required: ["type", "title", "description"]
+        properties:
+            type:
+                type: string
+                enum: [string]
+            title:
+                type: string
+            description:
+                type: string
+            example:
+                type: string
+            pattern:
+                type: string
+            format:
+                type: string
     BaseProvider:
       properties:
         name:
@@ -1813,9 +1831,19 @@ components:
           items:
             type: string
           type: array
+        connection_config:
+            $ref: '#/components/schemas/SimplifiedJSONSchema'
+        credentials:
+          type: object
+          properties:
+            apiKey:
+              $ref: '#/components/schemas/SimplifiedJSONSchema'
+
       required:
         - auth_mode
         - name
+        - display_name
+        - docs
       additionalProperties: false
       type: object
     ProviderOAuth2: