Enhanced termination predicates via CEL

In https://github.com/envoyproxy/nighthawk/pull/176 we landed the `termination-predicate` feature.
This allows configuration of simple chains of counter + thresholds combinations that will specify when and with which status an execution terminates by testing if thresholds are met.

However, during review @htuch mentioned [1] CEL (currently in use by Envoy in RBAC) as a potentially more powerful way to allow users to express more complex conditions. Leveraging CEL seems doable, and we could add it as a new `TerminationPredicate` implementation to make it co-exist with what we have today and stay backwards compatible.

Filing this issue to track this idea.

[1] https://github.com/envoyproxy/nighthawk/pull/176#discussion_r342899905