Allow WASM extensions configuration from secret

*Description*:
I'm currently using the `config` option for a Wasm extension. I have a sensitive value that I don't want explicitly defined in my chart or as a value. It would be great if I could configure the Wasm extension with a secret.

[optional *Relevant Links*:]
Wasm object definition: https://gateway.envoyproxy.io/docs/api/extension_types/#wasm