[TECH DEBT][etcd v2-v3] cache/etcd_cache.py - ensure functional tests & v3 compatibility

[waynew]

This is part of #60325

It's not blocked on deciding which library to use. The process is:

• - ensure that adequate functional tests exist for cache/etcd_cache.py
• - when a library is picked, update this module to take advantage of the
  etcdv3 library (if needed)

With adequate functional tests, we can ensure that when we drop in a new
library, our existing API doesn't need to change.