Support snap builders using a recipe

When defining a build pipeline for a package in Launchpad, the following four options are available:

- Build deb packages using a recipe (pull source, cherry-pick branches and then build). Uses the recipe syntax explained in https://help.launchpad.net/Packaging/SourceBuilds/Recipes
- Build snap packages (only pull source and build)
- Build charms (only pull source and build)

In the MicroK8s team, we are frequently building our snap for a number of different tracks, see https://launchpad.net/~microk8s-dev/+snaps

It would greatly help if we could somehow use the same recipe syntax when building some of our snap tracks. Is this something that would be easy to implement on LaunchPad?

For example, to understand the end goal of this ask, we would like to define recipes for building our 1.27-strict snap like this:

```
# git-build-recipe format 0.4 deb-version {debupstream}-0~{revtime}
lp:microk8s 1.27

merge strict lp:microk8s/strict
```

Which would translate to: use the 1.27 branch, and merge the strict branch before building.

Thanks!