Adding event extraction & formatter customization

[Al-un]

When lograge-sql works well when Lograge raw format is used, it might not be convenient to have a concatenated string when using Lograge JSON format. More globally, this suggestion allows a bit of flexibility:

• Log output can be customized via a formatter
• The formatter is fed by a event extractor. This allows user to extract additional fields (e.g. payload[:cached]
• Such customization are accessible via a Railtie. If no configuration is done, it works as currently defined and this change is completely invisible

_{Being a Ruby beginner, feel free to correct the code}

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For reference, ActiveRecord default event extractor and formatter:
https://www.rubydoc.info/gems/activerecord/ActiveRecord/LogSubscriber#sql-instance_method

[iMacTia]

Hi @Al-un and thanks for this massive contribution!
I like the customisation feature and I'm sure it will help a lot of developers.
I'm more than happy to merge it in and release a new version.