logging: add logger minimal severity

**Problem:**
All logs are sent to the Stackdriver, including debug logs. This is a problem for the security and privacy of logged data. Debug logs may contain sensitive data (for example: credit cards numbers or incoming request auth token).

**Solution:**
I suggest to add logger option `MinimalSeverity` (default value `Default`). `Logger.Log` and `Logger.LogSync` methods compare log entry severity with minimal severity. If entry severity lesser than minimal severity then log entry is ignored.

**Considered alternatives:**
I considered the option of creating my own wrapper over the logger, which would implement the above logic. But this logic is implemented in many well-known logging libraries (for example: zap or logrus), I decided to try to create this issue.