Customizable BPMN EventDefinitions and CMMN EventListeners

[filiphr]

Summary

Adds end-to-end extensibility for custom BPMN EventDefinitions and CMMN
EventListeners, and refactors the dispatch / deploy lifecycle / placement
validation paths around it so the built-ins go through the same code paths
as the customs.

What's new (extensibility surface)

• BPMN custom EventDefinitions: addCustomChildElementParser registers
  a parser at engine init so custom <flowable:*> children parse into a
  typed EventDefinition; addCustomEventDefinitionWriter registers the
  matching XML writer for round-trip serialization. A new
  CustomBpmnEventDefinition marker interface flags subclasses that route
  through the extension-elements path.
• CMMN custom EventListeners: a discriminator registry on
  GenericEventListenerXmlConverter keyed by flowable:eventType, with a
  customEventListenerTypeFactories slot on CmmnEngineConfiguration.
  flowable:eventType="event" is now a built-in discriminator value that
  produces an EventRegistryEventListener directly (alongside signal,
  variable, intent, reactivate); the legacy
  <flowable:eventType>X</flowable:eventType> extension element still
  works.
• EventDefinition#getSupportedLocations(): each subtype declares the
  immutable set of placements (start / event-sub-process start /
  intermediate catch / boundary / end / intermediate throw) it's valid at;
  all six placement validators consult this set instead of hard-coded
  instanceof chains.

Refactors that fall out of the design

• Behavior owns its full lifecycle:
  • Event-sub-process StartEvents implement
    EventSubProcessStartEventActivityBehavior and own
    initializeEventSubProcessStart(...). The five built-in registering
    behaviors (Message, Signal, Timer, EventRegistry, VariableListener)
    absorb the bodies of the corresponding handle... helpers in
    ProcessInstanceHelper, which collapses to a single delegated call.
  • Process-level StartEvents implement
    ProcessLevelStartEventActivityBehavior and own deploy(...) /
    undeploy(...). BpmnDeploymentHelper iterates start events; the
    EventSubscriptionManager and TimerManager classes are deleted.
  • DeploymentProcessDefinitionDeletionManagerImpl delegates restore-
    after-delete to behavior.deploy(...) with an
    isRestoringPreviousVersion() flag in the deploy context.
• Parse handler dispatch unification: StartEventParseHandler,
  IntermediateThrowEventParseHandler, and EndEventParseHandler no
  longer have instanceof cascades — each routes through
  parseElement(eventDefinition) like the boundary / catch handlers
  already did.
• CMMN parity: a CaseDefinitionStartLifecycleHandler attached to
  the Case at parse time mirrors the BPMN process-level pattern. The
  built-in EventRegistryCaseDefinitionStartLifecycleHandler is
  installed by default when case.startEventType is set.
• Bulk-flush event subscription cleanup: new
  findEventSubscriptionsByTypesAndProcessDefinitionId /
  findEventSubscriptionsByTypesAndScopeDefinitionId finders run a
  single SQL with EVENT_TYPE_ IN (...), so deploy-time cleanup is
  one DB round-trip regardless of how many handler types the previous
  definition declared.

Breaking changes

EventRegistryEventListener no longer extends GenericEventListener
(both extend EventListener directly now). Downstream code that does
instanceof GenericEventListener or
findPlanItemDefinitionsOfType(GenericEventListener.class) to enumerate
event-registry listeners must switch to EventRegistryEventListener (or
match both, where the runtime distinction matters).