allow content filterting

[aepfli]

https://docs.gradle.org/current/userguide/declaring_repositories.html#sec:repository-content-filtering

Currently we do not support contentfiltering, add functionality to support this

[kohenkatz]

Here is my workaround for anyone who is looking for this feature:

    maven {
        gitLab.project("ID_NUMBER").execute(this)
        content {
            includeGroup("com.example")
        }
    }

Basically, instead of passing the Action created by this plugin into the maven function directly, a new Action is created and the action returned by this plugin modifies it.