Recreate aiohttp ClientSession after DNS plug-in load (#5862)

* Recreate aiohttp ClientSession after DNS plug-in load

Create a temporary ClientSession early in case we need to load version
information from the internet. This doesn't use the final DNS setup
and hence might fail to load in certain situations since we don't have
the fallback mechanims in place yet. But if the DNS container image
is present, we'll continue the setup and load the DNS plug-in. We then
can recreate the ClientSession such that it uses the DNS plug-in.

This works around an issue with aiodns, which today doesn't reload
`resolv.conf` automatically when it changes. This lead to Supervisor
using the initial `resolv.conf` as created by Docker. It meant that
we did not use the DNS plug-in (and its fallback capabilities) in
Supervisor. Also it meant that changes to the DNS setup at runtime
did not propagate to the aiohttp ClientSession (as observed in #5332).

* Mock aiohttp.ClientSession for all tests

Currently in several places pytest actually uses the aiohttp
ClientSession and reaches out to the internet. This is not ideal
for unit tests and should be avoided.

This creates several new fixtures to aid this effort: The `websession`
fixture simply returns a mocked aiohttp.ClientSession, which can be
used whenever a function is tested which needs the global websession.

A separate new fixture to mock the connectivity check named
`supervisor_internet` since this is often used through the Job
decorator which require INTERNET_SYSTEM.

And the `mock_update_data` uses the already existing update json
test data from the fixture directory instead of loading the data
from the internet.

* Log ClientSession nameserver information

When recreating the aiohttp ClientSession, log information what
nameservers exactly are going to be used.

* Refuse ClientSession initialization when API is available

Previous attempts to reinitialize the ClientSession have shown
use of the ClientSession after it was closed due to API requets
being handled in parallel to the reinitialization (see #5851).
Make sure this is not possible by refusing to reinitialize the
ClientSession when the API is available.

* Fix pytests

Also sure we don't create aiohttp ClientSession objects unnecessarily.

* Apply suggestions from code review

Co-authored-by: Jan Čermák <sairon@users.noreply.github.com>

---------

Co-authored-by: Jan Čermák <sairon@users.noreply.github.com>

tests/api/test_store.py

@@ -2,7 +2,7 @@
 
 import asyncio
 from pathlib import Path
-from unittest.mock import MagicMock, PropertyMock, patch
+from unittest.mock import AsyncMock, MagicMock, PropertyMock, patch
 
 from aiohttp import ClientResponse
 from aiohttp.test_utils import TestClient
@@ -92,7 +92,9 @@ async def test_api_store_repositories_repository(
     assert result["data"]["slug"] == repository.slug
 
 
-async def test_api_store_add_repository(api_client: TestClient, coresys: CoreSys):
+async def test_api_store_add_repository(
+    api_client: TestClient, coresys: CoreSys, supervisor_internet: AsyncMock
+) -> None:
     """Test POST /store/repositories REST API."""
     with (
         patch("supervisor.store.repository.Repository.load", return_value=None),