Add property `variantName` to `KoverReport` interface

[shanshin]

What is your use-case and why do you need this feature? Property variantName will help to filter Kover tasks by selecting those that relate to specific variant.

So, to find the HTML report for debug Android build variant:

tasks.withType<KoverHtmlReport>().matching { 
    it.variantName == "debug" 
}