Add support for structured logging

[randomir]

As the use of Ocean/cloud-client in production app code (and/or server-side services code) increases, the need for easier parsed logs becomes apparent.

We need to:

• adapt log statements to log structured data
• abstain from configuring logging at all, and just dump to null handler
• make sure use of stdlib's logging and structlog is supported -- but defer actual configuration to the caller/app