Add tiny DSL for Extension configuration

[efenderbosch]

Nothing absolutely necessary, but having a DSL around the WireMockExtension.Builder would be nice.

fun wireMockExtension(block: WireMockExtension.Builder.() -> Unit): WireMockExtension =
    WireMockExtension.newInstance().apply(block).build()

Before:

companion object {
    @JvmField
    @RegisterExtension
    val wiremock: WireMockExtension = WireMockExtension.newInstance()
        .options(wireMockConfig().dynamicPort())
        .failOnUnmatchedRequests(true)
        .build()
}

After:

companion object {
    @JvmField
    @RegisterExtension
    val wiremock = wireMockExtension {
        options(wireMockConfig().dynamicPort())
        failOnUnmatchedRequests(true)
    }
}