Add wrapper to make wal-g more user-friendly

What

Add wrapper to make wal-g more user friendly.

Why?

wal-g must be called via /usr/bin/envdir /etc/wal-g.d/env /opt/wal-g/bin/wal-g and as the correct user. This is unintuitive and inefficient.

After this change users can just call gitlab-wal-g, analog to our other tools.