[Test proxy] Automate initial recording migration

Today, new libraries need to have their recordings manually pushed to the assets repo in order to generate an `assets.json` file. Instead of making library owners do this, since it's confusing and has caused issues in the past, we should automate this so that recordings are pushed if an `assets.json` file isn't present.

A potential issue with this would be permission-related, since manual recording migration requires some setup, but this configuration could just be specified as a basic requirement for testing.