fix(inventory): Fix inventory source caching

Fix inventory source caching by separating the fetching of objects and
populating the inventory. This way objects can be fetched from the K8S
API or from a configured cached and the cache related parameters on the
plugin now actually work.

The inventory source cache was tested with the ansible.builtin.jsonfile
cache plugin and 100k hosts (~2G JSON file). Though it took a noticeable
amount of time for the inventory plugin to run it worked fine and no
failures could be observed.

Signed-off-by: Felix Matouschek <fmatouschek@redhat.com>

examples/cache.kubevirt.yml

@@ -0,0 +1,5 @@
+---
+plugin: kubevirt.core.kubevirt
+cache: true
+cache_plugin: ansible.builtin.jsonfile
+cache_connection: kubevirt-cache