Add extdist module + role for labs

The extdist role sets up a tarball generator for use
with the Extension Distributor extension on mediawiki.org
running from labs.

The module can, in the future, be potentially used on prod.

Bug: 68609
Change-Id: I63d285612bd30d24b83718c4c8967144d146afeb
Merge "Add extdist module + role for labs" into production