[FLI-769] Allow for debug logging in engine

[markphelps]

We need a way to enable debug level logging for the engine, for example to see:

• what namespaces/flags the engine is aware of
• the results of requests to Flipt server
• the results of evaluation for variant/boolean

I think this will need to be implemented in two parts:

1. Add debug logging to the rust engine guarded
2. Add a way to configure this logging level per each client and pass that down to the rust engine

Instead of simply enabling DEBUG or not, we could think about how to support X number of log levels, like DEBUG, INFO, WARN, ERROR for example and standardize on those

_FLI-769