Come up with mechanisms of specifying/figuring out that some or all endpoints do not need to be migrated to the previous version

[zmievsa]

For example, if an endpoint was not directly affected, there are no request/response migrations for it, its dependencies didn't reference any schemas changed in the current version, its response model was not related to any changed schemas, and its body was not related to any changed schemas.
Additionally, you could do an easy shortcut for versions with purely side effect changes of skipping generation entirely.

This is all closely related to building a graph of changed schemas in #52 .