Print summary of discovered cache keys at end of session.

In the most basic of use cases, BuildStream will be able to derive what artifacts it will generate at the beginning of the build, so the cache keys will be visible in the opening summary.

However, when element tracking is enabled in the session, or when a workspace is open and active; cache keys can not be guessed until some activity in the session has occurred (finding a new ref, or hashing the result of a workspace build).

In these cases, it's difficult to see what exactly BuildStream built in the log file without browsing the huge log and searching for the resolved cache keys; these cache keys should be printed in the end summary in one place to make the log more informative of what was built.