feat: shorebird should support switching flutter versions

[felangel]

shorebird does not need to be tightly coupled to a single flutter release. We could offer fvm-like functionality, of downloading different versions of Flutter per project.

If we did this, then it would be possible for us to release a new version of Shorebird, which depend on a new version of Flutter without worrying that users then have to take explicit action to downgrade their version of Shorebird in order to manage older apps in the wild (patching already works this way but releasing currently requires being on the latest stable version).

Presumably if we did this we would only support a limited set of older flutter revisions, and only back to some certain point, as we do currently maintain (very small) patches on top of Flutter which we might have to also maintain on older branches of Flutter in this model.

[felangel]

related to #472