[R8] Add proguard config for android releases

[StefMa]

It seems that releases with R8 failing with

ERROR: R8: Missing class org.slf4j.impl.StaticLoggerBinder (referenced from: void org.slf4j.LoggerFactory.bind() and 3 other contexts)

This youtracks might be related to this:

• https://youtrack.jetbrains.com/issue/KTOR-3484/Add-generate-Proguard-script-for-Ktor-project-in-Gradle-Plugin
• https://youtrack.jetbrains.com/issue/KTOR-5528/Missing-class-warning-when-using-R8-with-ktor-client-in-android-application
• https://youtrack.jetbrains.com/issue/KTOR-7539/Embed-ProGuard-R8-rules

It seems it is safe to use

-dontwarn org.slf4j.impl.StaticLoggerBinder

But might be a little bit of deeper investigation if this is really okay or not 🙃