Allow ServiceFileTransformer to have its path be configurable

[aalmiray]

Currently the path 'META-INF/services' is hard-coded and private. The only way to configure a similar transformer while targeting a different path is to subclass the transformer (like so https://github.com/griffon/griffon/blob/development/subprojects/gradle-griffon-plugin/src/main/groovy/org/codehaus/griffon/gradle/shadow/transformers/GriffonFileTransformer.groovy).

It would be better if ServiceFileTransformer allowed its path to be configurable, like this

shadowJar {
    transform(ServiceFileTransformer) {
        path = 'META-INF/griffon'
    }
}

The current path value would be used as default though, so no additional configuration should be made if the standard location is the target path.