feat: add mode selecting (#15)

Co-authored-by: Timo Behrendt <t.behrendt@t00n.de>
Co-committed-by: Timo Behrendt <t.behrendt@t00n.de>

pkg/config/__mocks__/testLoadCanFindFile.yaml

@@ -1,16 +1,15 @@
----
 ip_provider:
   type: plain
   config:
-    url: https://ifconfig.me
+    url: https://example.com
 dns_provider:
   type: ionos
   config:
-    api_key: exampleAPIKey
+    api_key: exampleapikey
     base_url: https://example.com
 domains:
   - tld: example.com
     subdomains:
       - "@"
-      - www
-check_interval: 0 0 0/6 * * * *
+check_interval: 0 0 * * * *
+mode: RunOnce

pkg/config/__mocks__/testLoadInvalidMode.yaml

@@ -0,0 +1,2 @@
+mode: "InvalidMode"
+check_interval: "5m"

pkg/config/__mocks__/testLoadInvalidYAML.yaml

@@ -0,0 +1,3 @@
+mode: "Scheduled"
+check_interval: "5m"
+- invalid_content

pkg/config/__mocks__/testLoadMissingCheckInterval.yaml

@@ -0,0 +1,3 @@
+mode: "Scheduled"
+ip_provider:
+  type: "plain"