PoC: light-weight compile time dependency injection for logger

[justadreamer]

This PR accompanies the proposal: #4084. It demonstrates how different loggers can be easily swapped in compile-time, however leaving the possibility to also swap a global instance in runtime (during tests f.e.)

The key files are di/di.go, di/interfaces, di/providers

This is mainly to illustrate the point of the discussion and certainly not for immediate merging.

[hhhjort]

I think we want to add something about the compile time usage of this feature to the README. Mention the default, and how to compile the alternate logger in.

[justadreamer]

@bsardo mentioned that a similar thing is done with the time package - will take a look