Expose `reportDir` property in `KoverHtmlReport` interface

[shanshin]

What is your use-case and why do you need this feature?
if there is a need to analyze the HTML report after it is generated, or if you need to improve the report (for example, by editing CSS styles), you need to know the directory of each report.
Currently, it is not possible to easily link a specific report generation task and a directory.

Describe the solution you'd like
Add property reportDir interface kotlinx.kover.gradle.plugin.dsl.tasks.KoverHtmlReport

interface KoverHtmlReport: KoverReport {
    val reportDir: Provider<Directory>
}

Also, use case described in comment.

[shanshin]

Implemented in 0.8.0