[Feature Request] Support pre-invoke and post-invoke commands as DPkg::Pre-Invoke and DPkg::Post-Invoke in APT

[林博仁(Buo-ren, Lin)]

Etckeeper requires the software management system to run custom commands before and after an operation to allow auto-commiting changes in /etc caused by the software management system.

As an example, Etckeeper creates /etc/apt/apt.conf.d/05etckeeper with the following content to achieve this in APT:

```
DPkg::Pre-Invoke { "if [ -x /usr/bin/etckeeper ]; then etckeeper pre-install; fi"; };
DPkg::Post-Invoke { "if [ -x /usr/bin/etckeeper ]; then etckeeper post-install; fi"; };
```

Please support an equivalent mechanism so that changes in /etc made by Snappy can be easily distinguish from others

Refer this forum post for more info:
https://forum.snapcraft.io/t/counterpart-of-dpkg-pre-invoke-and-dpkg-post-invoke-configurations-in-snappy/2458

[r0mulux (r-marie)]

Hello,

I'm also very interested by such a feature, in particular execute a script after installation, upgrade, or removal of snap applications.

Regards,

[Achim Bohnet (allee)]

Having such a feature would be great! Keeping track of /etc/ changes with etckeeper starts to be a pain instead of the great helpful tool is was before snap.