OAuth2 configuration options (#123)

I opened up the OAuth2 authentication option. I needed this feature, and it is available in the gitea cli. So I opened it up for configuration through this pull request.

Hope it can help others.

Co-authored-by: Marc Went <marc.went+git@redkubes.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/123
Reviewed-by: luhahn <luhahn@noreply.gitea.io>
Reviewed-by: lafriks <lafriks@noreply.gitea.io>
Co-authored-by: Dunky13 <dunky13@noreply.gitea.io>
Co-committed-by: Dunky13 <dunky13@noreply.gitea.io>

values.yaml

@@ -131,6 +131,19 @@ gitea:
     #usernameAttribute: 
     #sshPublicKeyAttribute:
 
+  oauth:
+    enabled: false
+    #name:
+    #provider:
+    #key: 
+    #secret: 
+    #autoDiscoverUrl:
+    #useCustomUrls:
+    #customAuthUrl:
+    #customTokenUrl:
+    #customProfileUrl:
+    #customEmailUrl:
+
   config: {}
   #  APP_NAME: "Gitea: Git with a cup of tea"
   #  RUN_MODE: dev