Add hooks for pouring bottles.

This should give us a bit of control over what e.g. Boxen are doing
whilst at the same time stopping us from accidentally breaking each
other's stuff every so often.

I'm aware this may be somewhat controversial so I'm open to other
approaches.
Add tests for new bottling hooks.

Closes #27890.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>