API: return the api_version on the response (#10276)

* API: accept `X-RTD-Hosting-Integrations-API-Version`

This new header will force the API endpoint to return a specific JSON version
schema without taking into consideration the any other header.

The idea behind this is to allow theme authors guarrantees about the JSON scheme
they are expecting; without stopping us to move forward with potential changes
required to the API response when adding new features/addons/etc.

Related readthedocs/addons#64

* Remove old test

* Use text for `api_version`, since it could be something like `0.1.2`

readthedocs/proxito/tests/responses/v0.json

@@ -1,4 +1,5 @@
 {
+  "api_version": "0",
   "comment": "THIS RESPONSE IS IN ALPHA FOR TEST PURPOSES ONLY AND IT'S GOING TO CHANGE COMPLETELY -- DO NOT USE IT!",
   "projects": {
     "current": {

readthedocs/proxito/tests/responses/v1.json

@@ -1,3 +1,4 @@
 {
+    "api_version": "1",
     "comment": "Undefined yet. Use v0 for now"
 }

readthedocs/proxito/views/hosting.py

@@ -244,6 +244,7 @@ def _v0(self, project, version, build, filename, user):
         project_features = project.features.all().values_list("feature_id", flat=True)
 
         data = {
+            "api_version": "0",
             "comment": (
                 "THIS RESPONSE IS IN ALPHA FOR TEST PURPOSES ONLY"
                 " AND IT'S GOING TO CHANGE COMPLETELY -- DO NOT USE IT!"
@@ -432,6 +433,7 @@ def _v0(self, project, version, build, filename, user):
 
     def _v1(self, project, version, build, filename, user):
         return {
+            "api_version": "1",
             "comment": "Undefined yet. Use v0 for now",
         }