Handle community packs

[SimonMellerin]

The idea is to let people create their own packs of anonymizers and to distribute them via composer/packagist.

First, we will assume that each one of these packs will be created from a given template (provided by us).

This template will bootstrap a compose.json that will define a specific type : db-tools-bundle-pack

Then, the DbToolsBundle should be able to find these packs and to configure himself to use them. This will be done with a command : db-tools:discover-pack.

This command will scan each package in the vendor directory to find those with type db-tools-bundle-pack. Then it will add paths to the src directory of these packs to the anonymizer_paths configuration.

In order to avoid to interfere with user defined configurations, configuration declared by this command will be add to a config/db_tools_pack.yaml file.

In a second time, the idea is to use a Symfony Recipe to add this command to be launched automatically by composer.

[SimonMellerin]

Finally, for now, we will locate pack directly in src/Anonymization/Anonymizer/AnonymizerRegistry.php at runtine.

This will be much more simple, we won't need Symfony recipe or a specific command for that.

Ideally, this routine should be cached and initialize at warmup (see #26) and not at runetime. But, it's not such a big deal for now: this service AnonymizerRegistry will only be constructed while using one of the commands provided by this bundle.