Add context.message to the Intake API for span and transaction and map to appropriate ES fields.

• context.message.queue.name
• context.message.topic.name
• context.message.body
• context.message.headers

more details in elastic/apm#143

Things to clarify:

• sent for transaction only or also for span events? (see Add messaging fields to schema apm#143 (comment))
• Fields in ES: advise with ECS team on naming, even when not directly added to ECS, ensure the fields are chosen in a way to not conflict with future developments in ECS.
• Do any of the fields need to be indexed, if so, which ones? Clarify the use cases for the information.