Allow steps to be run inside a container/distrobox

[sunnyayyl]

Add configuration to allow steps to be ran inside a distrobox

I am using a atomic/rpm-ostree system
Especially for cargo-install-update, dependence of some of the cargo crate might require system dependencies (e.g. probe-rs-tools), which would usually be installed in a distrobox instead, so it would be nice if it's possible to configure topgrade to run steps within a distrobox

• Which exact commands should Topgrade run?
  distrobox-enter {container-name} -e {command of the step}