feat: Allow for multiple MessageBus subscriptions

[lenny-goodell]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Only one topic allowed for MessageBus trigger

Issue Number: #596

What is the new behavior?

Multiple topics allowed for the MessageBus trigger

Does this PR introduce a breaking change?

• Yes
• No

Are there any new imports or modules? If so, what are they used for and why?

no

Are there any specific instructions or things that should be known prior to reviewing?

Other information

[codecov-io]

Codecov Report

Merging #625 (ca4780b) into master (baae3ee) will increase coverage by 0.01%.
The diff coverage is 70.68%.

@@            Coverage Diff             @@
##           master     #625      +/-   ##
==========================================
+ Coverage   60.81%   60.83%   +0.01%     
==========================================
  Files          31       31              
  Lines        1807     1823      +16     
==========================================
+ Hits         1099     1109      +10     
- Misses        635      640       +5     
- Partials       73       74       +1     

Impacted Files	Coverage Δ
appsdk/sdk.go	33.33% <0.00%> (ø)
internal/trigger/messagebus/messaging.go	70.58% <73.21%> (-1.88%)	⬇️

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