Generic way for configuring Gitea app.ini (#240)

With the result of PR #239 it is much easier to provide additional values to the _app.ini_ configuration from different sources.
These changes adds an _additionalConfigSources_ field where the users can define such sources. This enables the users to choose
on their own whether to store values in _values.yaml_ or load them from Kuberetes Secrets or ConfigMaps.

- Fixes #243
- Fixes #174
- Fixes #260

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/240
Reviewed-by: luhahn <luhahn@noreply.gitea.io>
Reviewed-by: wxiaoguang <wxiaoguang@noreply.gitea.io>
Co-authored-by: justusbunsi <justusbunsi@noreply.gitea.io>
Co-committed-by: justusbunsi <justusbunsi@noreply.gitea.io>

values.yaml

@@ -205,6 +205,12 @@ gitea:
   #  security:
   #    PASSWORD_COMPLEXITY: spec
 
+  additionalConfigSources: []
+  #   - secret:
+  #       secretName: gitea-app-ini-oauth
+  #   - configMap:
+  #       name: gitea-app-ini-plaintext
+
   podAnnotations: {}
 
   database: