[FEA] Support specifying a log level in runtime

[gigony]

Is your feature request related to a problem? Please describe. It would be nice if cuCIM provides a way to specify log levels in runtime so that users can set a log level to see log messages selectively.

This solves the following issue in a general way.

• https://github.com/rapidsai/cucim/issues/109

Describe the solution you'd like

• Accept environment variable to specify log level
• Provide API to get/set a log level

Additional context

• https://github.com/rapidsai/cucim/issues/109