feat: add new 'Topics' field and external MQTT BootstrapHandler

[chr1shung]

to support north-south messaging implementation for internal and
external MessageBus connection

Signed-off-by: Chris Hung [email protected]

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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?) new handler depends on mqtt-broker, haven't figured out how to test it
• 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?) wait until the whole implementation is finished

Testing Instructions

New Dependency Instructions (If applicable)

https://github.com/eclipse/paho.mqtt.golang
used by app-functions-sdk-go to connect to external MQTT broker.
We extract the logic here so that it can be reused by north-south messaging implementation and app-sdk in the future.

[codecov-commenter]

Codecov Report

Merging #365 (0669cbd) into main (8679ad0) will decrease coverage by 3.80%.
The diff coverage is 1.03%.

@@            Coverage Diff             @@
##             main     #365      +/-   ##
==========================================
- Coverage   61.52%   57.72%   -3.81%     
==========================================
  Files          23       24       +1     
  Lines        1458     1554      +96     
==========================================
  Hits          897      897              
- Misses        523      619      +96     
  Partials       38       38              

Impacted Files	Coverage Δ
bootstrap/handlers/externalmqtt.go	0.00% <0.00%> (ø)
config/types.go	35.29% <ø> (ø)
bootstrap/handlers/messaging.go	83.33% <100.00%> (ø)

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[lenny-goodell]

LGTM