Add Amazon MQ for RabbitMQ event structure

[Infra-Red]

Description of changes:

Adds Amazon MQ for RabbitMQ event structures.

• An example event was taken from https://docs.aws.amazon.com/lambda/latest/dg/with-mq.html#rabbitmq.

> go test -race  ./events
ok      github.com/aws/aws-lambda-go/events     0.605s

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[Infra-Red]

@bmoffatt Could you please review this change. Thanks!

[bmoffatt]

Id -> ID everywhere

Really gotta get around to adding a linter rule for this, that's on me :)

[bmoffatt]

note to self to double check the docs - sample data makes it seem like this almost could have been defined as

type RabbitMQBasicProperties struct {
  // ...
  Headers map[string]RabbitMQBasicPropertiesHeaders `json:"headers"`
  // ...
}
type RabbitMQBasicPropertiesHeaders struct {
  Bytes []byte `json:"bytes"`
}

and it'd be nice if possible if a user didn't have to drop down into having to reflect over an interface{} value

[bmoffatt]

I see now that the sample data was copy+paste from the docs, so I guess there's not much that can be done here

[codecov-commenter]

Codecov Report

Merging #387 (5603671) into main (4005b1d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #387   +/-   ##
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  Coverage   71.63%   71.63%           
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  Files          19       19           
  Lines        1040     1040           
=======================================
  Hits          745      745           
  Misses        228      228           
  Partials       67       67           

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