Feature request: Add logging API

[viccuad]

Logging is always useful when developing, debugging, or understanding complex policy executions.

Acceptance criteria

• Since the JS SDK is using WASI (that is, interfacing with the Wasm host via Stdin, Stdout and stderr), policies cannot print logs to stdout or stderr. The logs need to use WASI logging interfaces.
• Come up with a simple SDK WASI logging API and expose it.
• Consume the API in the demo policy and the template, to showcase its use.