Support for creating/updating config files

Added support to create and update config files for the Config File Provider plugin.

New DSL syntax:

```groovy
configFile {
    name 'my-config'
    comment 'My important configuration'
    content '<some-xml/>'
}

configFile(type: MavenSettings) {
    name 'central-mirror'
    content readFileFromWorkspace('maven-settings/central-mirror.xml')
}
```