Introduce common logging policy

[YuriyVelichkoPI]

Motivation

Prebid SDK's logs are reasonably rotten. It is hard to read and understand them. The verbose level is often mismatched. There is no consistency between iOS and Android logs.

Objectives

• Design the common logging policy for Prebid SDK
• Discuss and approve the policy with the community (TODO: create the subtask once the work is approved)
• Implement the policy on both platforms (TODO: create the subtasks for the platforms due to the approved policy)

[bretg]

Would also prefer for the log levels to be consistent across the platforms

• ios log levels can be .error, .debug, .verbose, .warn, .severe, and .info.
• android supports only ERROR, DEBUG, WARN, and NONE