Add and propagate message attributes

[adenhertog]

Message attributes (aka headers) allow metadata to be passed around the system without explicitly having to be included in the message body. Common use cases for attributes are correlation ids, identification of the source trigger/user of a set of message flows and other contextual information.

This enhancement aims to achieve:

• An easy way to pass message attributes when sending a message
• Ability to automatically propagate attributes to subsequent messages (ie: in the case of workflow handling scopes, message handlers etc)
• Ability to remove or modify an attribute

[adenhertog]

Implemented in 596c192