Add commands for inspecting configured listeners

[HypeMC]

Similar to Symfony's debug:event-dispatcher command, this PR adds two new commands to inspect configured Doctrine event/entity listeners:

• doctrine:debug:event-manager — lists event listeners registered in the Doctrine EventManager for a given entity manager (optionally filter by event).
• doctrine:debug:entity-listeners — lists entity listeners configured for a given entity class (optionally filter by event).

[greg0ire]

Hi! Thanks for your contribution. Did you consider contributing this directly to doctrine/orm? If yes, what made you choose this package instead?

[HypeMC]

@greg0ire Hi. Yes, I considered doctrine/orm, but I wasn't sure which repo was the best fit. I picked this one because in Symfony apps listeners can be configured in various places (attributes, YAML/XML, etc.), so a debug command here feels most useful. If you prefer it in ORM, I'm happy to open a PR there instead.

[ostrolucky]

yes please doctrine/orm