Provide 'rust' logging API

[flavio]

A policy author should be able to generate logs from within a policy.

These log messages are going to be picked by the waPC host, which will process them according to its configuration.

This is part of kubewarden/policy-server#73

Acceptance criteria

• User can use simple string messages
• User can do structured logging if wanted: simple string message + dictionary with key/values
• The following log levels are available: debug, info, warning, error