Create example of Wasm Singleton

[mandarjog]

Wasm singletons are used to build state centrally, and example that shows how to do the following

1. Populate shared state that can be used from normal filters.
2. Show how this state can be updated, either thru XDS or some other means.

cc @bianpengyuan

@mdhume since you use bootstrap extension, do you mind contributing an example plugin?

[mdhume]

Sure @mandarjog we can take this up