Run Gradle plugin extensions

[chanseokoh]

Gradle counterpart of #2397.

Using the standard Service Provider Interfaces (SPI) mechanism to load and run Gradle plugin extensions.

This doesn't yet involve the new extension config option (like jib.extensions=[ { implementation='com.example.ExtensionClass' }]). The config option can be added in a later PR.

With this, we are actually enabling running and testing Gradle extensions. Extensions should be added to the classpath of the Gradle build script. Extensions added in this way apply to all sub-modules in a multi-module project, so we do need the new config option in the end.

buildscript {
    dependencies {
        classpath 'com.google.cloud.tools.jib.plugins.extension.gradle.quarkus:quarkus-extension-gradle:0.1.0-SNAPSHOT'
    }
}