Pass invocation parameters to custom commands

[Laura7089]

It would be a convenient feature if topgrade informed the {pre_,,post_}commands scripts of the configuration topgrade was invoked with.

My initial thoughts are environment variables like TOPGRADE_CLEAN=1 and TOPGRADE_ACCEPT_ALL=1 for the clean = true config option and -y CLI flag respectively.

My motivation for this is primarily that I have a couple of custom commands which implement cleaning processes that probably shouldn't be upstreamed into topgrade itself since they're quite opinionated. For example:

• I clean out all automatically-installed packages which no longer have any dependents
• I clean podman storage with podman prune -af.

I'd like to make these "steps" conditional on the clean config option.

[Laura7089]

I'm happy to PR this feature myself, but I wanted to hear maintainer thoughts first.