Migrate sourcesecret package to runtime/secrets APIs

The sourcesecret package now uses pkg/runtime/secrets factory
functions instead of the previous monolithic approach. This
provides standardized secret generation with consistent
validation and error handling across all authentication types.

Signed-off-by: cappyzawa <cappyzawa@gmail.com>

cmd/flux/testdata/create_secret/helm/secret-helm.yaml

@@ -7,4 +7,5 @@ metadata:
 stringData:
   password: my-password
   username: my-username
+type: kubernetes.io/basic-auth