ssbc: Build changeset specs server side from cache entries
Created by: eseliger
Requires https://github.com/sourcegraph/src-cli/pull/661
This PR closes the bonus goal in EM2.3 to move changeset spec generation into the backend.
After this change, src-cli doesn't need to build the changeset specs anymore and also doesn't need to create them via the API. We will just extract the cache entries from the logs and build the changeset specs based on those, which is 100% equivalent and, besides repo archive fetching, removes the need of src in executors to talk to the API at all.
Closes #26142 (closed)