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/proxy/secret-proxy.yaml

@@ -8,4 +8,5 @@ stringData:
   address: https://my-proxy.com
   password: my-password
   username: my-username
+type: Opaque