mirror: add new endpoint to submit mirror without marking configured

The POST handler for /mirror applies the URL passed but also marks the
model configured. That said, we want the ability to:

 1. supply a mirror URL
 2. run the mirror test
 3. mark the model configured if the test is successful

This patches creates a new endpoint: /mirror/candidate that does the
same as /mirror except that it does not mark the model configured.

It is therefore suitable for step 1.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

subiquity/common/apidef.py

@@ -345,6 +345,9 @@ class API:
         def GET() -> str: ...
         def POST(data: Payload[str]): ...
 
+        class candidate:
+            def POST(url: Payload[str]) -> None: ...
+
         class disable_components:
             def GET() -> List[str]: ...
             def POST(data: Payload[List[str]]): ...

subiquity/server/controllers/mirror.py

@@ -177,6 +177,10 @@ class MirrorController(SubiquityController):
         self.model.set_mirror(data)
         await self.configured()
 
+    async def candidate_POST(self, url: str) -> None:
+        log.debug(url)
+        self.model.set_mirror(url)
+
     async def disable_components_GET(self) -> List[str]:
         return sorted(self.model.disabled_components)