Use swift-log as logging system

[filipbodor]

As we now use swift-log in slido for mac project, it makes sense to transfer to the logging system also in this swift library.

Removed:
I removed old logging system which was used by this library.

Implemented:
Instead of storing OAuth2Logger instance, the library will store Logger instance from swift-log framework.
I needed to change the way log messages were written:

logger.warn()                                 // -> logger.warning()
logger.info("OAuth2", msg: "message")         // -> logger.info("message")

[DominikPalo]

I'd change the wording to "logging level" since we no longer pass a logger instance here:

Suggested change

	Need to debug? Use a `.debug` or even a `.trace` logger:
	Need to debug? Use a `.debug` or even a `.trace` logging level:

Also, I'd probably add a small note or section mentioning that the library uses the Swift logging system, and include a link to its homepage: https://swiftpackageindex.com/apple/swift-log