@Clock, @EventProcessing in RuleUnit

Issue Description:

@Clock, @EventProcessing seem to work in Kogito but not in pure Drools RuleUnit.

Acceptance Criteria:

Add unit tests for them and fix if needed.

Also add to docs.

Additional Information:

@SessionsPool can be technically conveyed to KieBase but actually KieSessionsPool is not used because RuleUnit creates RuleUnitInstance backed by RuleUnitExecutorImpl, not KieSession. We may introduce another pool mechanism for RuleUnitInstance, but RuleUnitExecutorImpl is lightweight than KieSession so it may not be highly demanded.