AMQP-428: Add Logback `AmqpAppender`

JIRA: https://jira.spring.io/browse/AMQP-433, https://jira.spring.io/browse/AMQP-428

Fix `amqp.rabbit.log4j.AmqpAppender` JavaDoc typo

Polishing

Add category abbreviation [o.s.a.r.l.AmqpAppenderIntegrationTests].

Retain full category in AMQP headers.