Issue #192 | Adding a MessageUUIDHeaderKey configuration

[Agronis]

Allows users of AMQP subscriber to not be bound to an upstream AMQP watermill publisher.

Reference ThreeDotsLabs/watermill#192

[roblaszczak]

Hello @Agronis. To be honest I missed this PR (:man_facepalming:) and prepared mine (#4), which removes the requirement to have UUID in the message.

If you feel that auto-generation of UUID may be useful, you can align this PR to mine changes. My only suggestion will be:

• ExpectMessageUUIDHeader - can probably describe better, that it will auto-generate message UUID
• _, err := uuid.Parse(msgUUIDStr) - message UUID don't need to be a valid uuid! so you can remove that.

Cheers!