Feature: Built-in check for Symfony Messenger queue size

[rothfahl]

This adds the ability to configure checks, that ensure that the count of messages in Symfony Messenger transports does not exceed expected limits. It uses MessageCountAwareInterface and therefore only works for transports that implement that interface.

Dear maintainers, we implemented similar checks in several projects. I've now ported that into a feature on library level. Please let me know, if you don't consider it as in scope of this project or if i need to add/change anything before it can be merged.

[kbond]

Don't worry about the CI problems. I'll have a look after merging.

Do you mind updating the config docs here: https://github.com/liip/LiipMonitorBundle#full-default-config

[kbond]

Thanks @rothfahl!

[rothfahl]

thank you @kbond!