Support download of artifacts with classifier

Beside the main artifact there can be additional files which are
attached to a project. Such attached files can be recognized and
accessed by their classifier.

Extend the `maven_jar` wrapper to support optionally specifying
the classifier in the `id` field, for example:

  maven_jar(
    name = 'example',
    id = 'org.example:example-foo:1.0:extra',
    ...,
  )

Change-Id: Ic0a4fbafffe6625b21324e836cde1a2b3a3e4b1c