[#13] Implemented Mail Gateway Processor

.env

.env-mail-gateway

.flake8

.isort.cfg

Makefile

docker-compose.yml

docker/activemq/mail_gateway_consumer.conf

docker/activemq/mail_gateway_publisher.conf

notifications_consumer/config.py

notifications_consumer/consumer.py

notifications_consumer/data_source/data_source.py

...ons_consumer/data_source/postgres/postgres_data_source.py

notifications_consumer/processors/__init__.py

notifications_consumer/processors/mail_gateway/exceptions.py

notifications_consumer/processors/mail_gateway/processor.py

notifications_consumer/processors/mail_gateway/utils.py

notifications_consumer/processors/processor.py

notifications_consumer/processors/registry.py

notifications_consumer/publisher.py

notifications_consumer/utils.py