feat: beet.contrib.bake_overlays

[rx-dev]

Adds beet.contrib.bake_overlays as a new beet plugin to help collapse overlays onto a singular version.

open questions

• this only handles namespace and ignores extras and namespace_extras
• the naming of the config is a bit odd
• if config isn't available, it pulls from the set pack.pack_format which usually pulls from the latest version (actually, if set in beet.json, if a pack.mcmeta exists, it'll still be the latest if it's still unset).
• the merging temporarily replaces a merge policy with a defunct one. unsure if this is the best route for all merging involved.
• Make plugin also work for generic packs and not just ctx