Support for excluding private methods in utbot-gradle #482

Description

Out gradle plugin collects all declared methods from the class. But sometimes there is no need to test private methods. So we should add a field testPrivateMethods: Boolean to the plugin configuration.

Expected behavior

The user can configure the plugin:

sarifReport {
    testPrivateMethods = false
}

or just execute the task:

generateTestsAndSarifReport -PtestPrivateMethods='false'

Context

Gradle plugin documentation: utbot-gradle/docs/utbot-gradle.md