Allow WASM extensions configuration from secret

[daniel-white]

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

[zhaohuabing]

Ideally the sensitive value should be a Secret reference in the config, and only the Wasm module should be able to access it.