Add ability to promote packages (incl deps)

[sunnyflunk]

One of the last standing issues with getting packages to users is now security updates outside of the standard repo syncs, or quick fixes to problems not spotted prior to sync e.g. https://dev.solus-project.com/T4575

• Seeing the full list of promoted packages before hand would be important, so we can validate the revdeps are safe to push (where the full repo may not be properly tested or in a sync state). It maybe that waiting for a full sync is a better option in some circumstances.
• Validating the repo (all packages can be installed, including release numbers), so a partial push doesn't create issues. Whether this is a part of ferryd or elsewhere.

[ikeydoherty]

So we could have a ferryctl command to do this, and a kinda "dry run" operation which'd show the impact of the include, and what it depends on in turn. We store the dependencies inside the database so we're able to resolve this without a DB change.