Mautic project: Enhance bounce management capabilities

Mautic processing of email webhooks from Omnivery, Mailgun and similar Email Service Providers is a bit limited due to insufficient underlying model where we store information coming from webhooks. This proposal aims to add ability to the Mautic core in a way so that transport plugins can leverage them and map it more precisely to the Mautic core model. This will allow better handling of bounces, complaints and other email events, leading to improved deliverability and sender reputation for Mautic users. For the problem described above we're suggesting that we add a complaint flag into the DNC entity so transport plugins can use it when the Email Service Provider suggests so. We would also like to introduce soft bounces into Mautic core in a way so that we still send to contacts that soft bounces, but if the soft bounce happens multiple times that translates to the DNC entity for that lead with a hard bounced flag. How many times contact soft bounces consecutively before it translates to hard bounced will be configurable with the setting in Configuration -> Email Settings. We introduce soft bounce categories so that transport plugins can categorize and we can display that data to the end user. We would add the following filters on segments: filter for DNC complaint reason, soft bounce filter and soft bounce of specific category. We would like to show some more statistics about bounced, soft bounced and complaint contacts on the email list page and to show the count of soft bounces & complaints on the contact page in addition to showing how many soft bounces are left until the hard bounce is set for the given contact. We'll show the count of soft bounces & complaints on email list view. This will allow marketers to have better insights into their email campaigns and take necessary actions to improve deliverability.