Support shared configurations

[raniejade]

Specs can be configured by overriding configure:

class SomeSpec: KSpec() {
    override fun configure(config: KSpecConfig) {
        ...
    }
    ...
}

Configuration can be shared via inheritance however it's not that flexible. Allow specifying configurations via an annotation (@Configurations) which can contains an array of Configuration classes, e.g:

Configuration

class SharedConfiguration: Configuration {
    override fun apply(config: KSpecConfig) {
        ...
    }
}

Spec

@Configurations(SharedConfiguration::class)
class SomeSpec: KSpec() {
    ...
}

Configurations are applied in the order they are listed.