Add support + test for dynamic extensions amps

[rschev]

Basic support for making a dynamic extensions amp by leveraging the existing dynamic extensions gradle plugin and just embedding the resulting jar in the amp. The syntax for this in the build.gradle is:

amp {
    de {
        from project.jar
    }
}

[rschev]

Changed your mind about adding a new CopySpec in the getter?

[vierbergenlars]

@rschev Yes, because when you change more settings of the copyspec, you don't want to change previous copyspecs.

Example:

amp {
  de {
    from configurations.runtime
  }
  de {
    from project.jar
    rename { "project-module.jar" }
  }
}

If it was the same copyspec, this would rename all jars from configurations.runtime to project-module.jar, which is quite unexpected behavior.