feat: Add Will configuration elements for External MQTT trigger config

[lenny-goodell]

closes #1117

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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?) N/A
• 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?)
  TBD one refactor PR is merged

Testing Instructions

Run edgex Stack

 make run mqtt-broker mqtt-verbose no-secty

Build ASC with the SDK from this branch
Modified external-mqtt-trigger profile to add the following to Trigger.ExternalMqtt section

    Will:
      Enabled: true
      Payload: "goodbye"
      Qos: 2
      Retained: true
      Topic: "me/be/gone"

Run ASC

./app-service-configurable -d -cp -p=external-mqtt-trigger

Verify the following is in log file.

msg="Last Will options set for MQTT Trigger: {Enabled:true Payload:goodbye Qos:2 Retained:true Topic:me/be/gone}"

Verify logs for the Mqtt Broker contain

Will message specified (7 bytes) 

Connect mqtt.FX to MQTT Broker ad subscribe to me/be/gone topic
Stop ASC
Verify mqtt.FX received goodbye message on me/be/gone topic

New Dependency Instructions (If applicable)

[codecov-commenter]

Codecov Report

Merging #1493 (053e043) into main (2743bc4) will decrease coverage by 0.09%.
The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##             main    #1493      +/-   ##
==========================================
- Coverage   67.18%   67.09%   -0.09%     
==========================================
  Files          36       36              
  Lines        3093     3097       +4     
==========================================
  Hits         2078     2078              
- Misses        881      885       +4     
  Partials      134      134              

Files	Coverage Δ
internal/trigger/mqtt/mqtt.go	36.36% <0.00%> (-1.14%)	⬇️

[jim-wang-intel]

LGTM