Allow event subscriptions query in the Engine

Context
Some customers want to use BPMN as a substitute for CMMN. As such, they want to be able to query event subscriptions. This query result might be used to show which messages can a user consume to spawn dynamic tasks.

Currently, this query is not possible through the REST API. The current resource allows only to see a certain event subscription tied a message name and an execution: https://docs.camunda.org/manual/latest/reference/rest/execution/message-event-subscription/get-message-subscription/

Propostal

Create a resource event-subscriptions and allow queries to it (Get List) with the same query criteria that we have in the Java API.

Background / example usage
The customer's UI allows to interact with a given process instance from within a task but also from outside the instance. Tasks and embedded sub processes are used to scope message events which allow e.g. to reset or cancel parts of the process. They also use event sub processes which "switched on" a message received event via a conditional trigger. The event sub process then contains an event-based gateway, which allows to receive a cancellation message or end the event sub process. as shown in screenshot-1. The UI needs to be able to query what message events, be it intermediate or boundary events on tasks, embedded sub processes or in the main process are currently active, so the UI can show the relevant actions to the user.