feat: Add external-mqtt-trigger configuration file for TAF test

[cherrycl]

Add external-mqtt-trigger for edgexfoundry/edgex-taf issue #514.

Signed-off-by: Cherry Wang cherry@iotechsys.com

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PR Checklist

Please check if your PR fulfills the following requirements:

• I am not introducing a breaking change (if you are, flag in conventional commit message with BREAKING CHANGE: describing the break)
• I am not introducing a new dependency (add notes below if you are)
• I have added unit tests for the new feature or bug fix (if not, why?)
• I have fully tested (add details below) this the new feature or bug fix (if not, why?)
• I have opened a PR for the related docs change (if not, why?)

Testing Instructions

1. Deploy app-service with external-mqtt-trigger profile and mqtt-broker.
2. Subscribe both topics "external-request": SubscribeTopics in Trigger and "edgex-export": Topic in Pipeline Function.
3. Publish a message "test message" to the topic "external-request" which is defined in the profile.
4. Both topics, external-request and edgex-export, receive the message

New Dependency Instructions (If applicable)

[lenny-goodell]

LGTM