Exposing the API to collect all apps

Adding the api that returns all apps from the database to the client
to be used in the 21-k8s-app-upgrade script in order to correctly log
the current update/revert state of the apps.

Test Plan:
PASS: build-pkgs && build-image
PASS: Fresh install AIO-SX
PASS: Platform upgrade stx-10 to master
PASS: Platform rollback from master to stx-10
PASS: The api returns a list of apps loaded into the database as
      expected.

Partial-bug: 2121827

Change-Id: I1e1d6e653e7bc0f0a529413b7c31996dff7d95ef
Signed-off-by: David Bastos <david.barbosabastos@windriver.com>

sysinv/cgts-client/cgts-client/cgtsclient/v1/kube_app.py

@@ -15,6 +15,10 @@ class KubeAppManager(base.Manager):
         _, body = self.api.json_request('POST', f"{self.path}evaluate_apps_reapply/", body=triggers)
         return body
 
+    def get_all_apps(self):
+        _, response = self.api.json_request('GET', f"{self.path}apps/")
+        return response.get('apps', [])
+
     def update_all(self):
         return self.api.json_request('POST', f"{self.path}apps/update_all/")