Distributor: Consume events that signal a subscription change

[warber]

Action Item

In KEP52 the proposal is, the give the user the opportunity to customize subscriptions for a given Keptn Integration.

Supporting this use case, the distributor needs to be triggered to update its current subscriptions to the desired state.
In order to do so, the distributor listens for and reacts to the general purpose event (e.g. sh.keptn.event.subscription.changed, defined in #4437 ) which holds the current subscription information for a particular keptn integration and updates its subscriptions accordingly.

ℹ️ In order to implement the above feature, also a distributor which is running on a remote execution plane needs to be able to receive event other than *.triggered. (done in #4498)

Acceptance Criteria

• Keptn integration can be used to update its subscriptions dynamically.
• Distributor is able to create subscriptions with different filters each
• Solution is backwards compatible

Definition of Done

• existing integration test is enhanced to test this functionality
• documentation updated

Dependencies

#4437
#4498

[thisthat]

With #4987, we decided to implement this feature using APIs instead of events.