NGSTACK-805: async indexing

[pspanja]

This implements support for asynchronous indexing through Symfony's Messenger component. Implementation of indexing event listeners from Ibexa core is used as a base for this implementation, but it's improved to take into account the possibility of failure as a part of normal operation. It's mostly about handling the case when Content/Location that should be processed was deleted and hence can't be loaded from the Repository at the time of processing.

Example configuration for the Messenger component is here given as a reference point.

In order to enable asynchronous indexing in a project, a following configuration is needed:

netgen_ibexa_search_extra:
    use_asynchronous_indexing: true

Additionally a consumer processing the asynchronous indexing queue should be running, with the provided example configuration:

bin/console messenger:consume netgen_ibexa_asynchronous_indexing

Indexing command is not modified, meaning it works aside from the asynchronous mechanism.

[pspanja]

Thanks @emodric and @igorkrz!