Add support for Amazon MQ (broker + configuration)

[radeksimko]

Depends on #2465 (I'll rebase this PR after merging)

Also I'm breaching the etiquette here a little by submitting multiple new resources in a single PR, but the code was already more-or-less reviewed in a private fork where we were developing this prior to public announcement of the service yesterday at re:invent.

Test results

TF_ACC=1 go test ./aws -v -run=TestAccAwsMq -timeout 120m
=== RUN   TestAccAwsMqBroker_basic
--- PASS: TestAccAwsMqBroker_basic (1145.34s)
=== RUN   TestAccAwsMqBroker_allFields
--- PASS: TestAccAwsMqBroker_allFields (2114.92s)
=== RUN   TestAccAwsMqBroker_updateUsers
--- PASS: TestAccAwsMqBroker_updateUsers (1568.25s)
=== RUN   TestAccAwsMqConfiguration_basic
--- PASS: TestAccAwsMqConfiguration_basic (49.94s)
=== RUN   TestAccAwsMqConfiguration_withData
--- PASS: TestAccAwsMqConfiguration_withData (21.10s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	4899.602s

[catsby]

👍 pending rebase on that other PR and the vendor changes here go away with it

[ghost]

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