Add option to configure the verbosity of the logs

[ignazioc]

Hi,
The current logs produced by the SDK are pretty verbose. This is great during the debug but during the every day job is too much noise that hides the real important messages.

With this PR I added an option (https://github.com/technology-ebay-de/prebid-mobile-ios/blob/9d723ea3d07800a13c9d3a34382b3e9b567c0825/src/PrebidMobile/PrebidMobile/Prebid.swift#L53) to allow the clients to choose a different LogLevel.

All the logs, are anyway disabled on release builds.

The default of this value is debug to do not change the current behaviour.

For the level of verbosity I reused the existing enum. This enum is sorted based on the verbosity of the log. .debug will print everything, severe only the severe error.

public enum LogLevel: String {
    case debug = "[💬]" // debug
    case verbose = "[🔬]" // verbose
    case info = "[ℹ️]" // info
    case warn = "[⚠️]" // warning
    case error = "[‼️]" // error
    case severe = "[🔥]" // severe
}