[identity] Move token definitions out of token cache to generify token cache interface

This refactors the token cache interface to separate token definitions
and cache logic.
Token and key definitions are pulled out of the token cache and moved
to token manager.
The cache provides traits that should be implemented by anything
stored in the cache.
This also adds test configuration to token manager to support tests
migrated from token cache.

Next changes: Enforce maximum cache size.

Change-Id: I63b403a2192e1cdd6b8f1f36d7cb8b334f9a9829