feat: expose processed message's credential with its associated public signature key [WPB-5975]

[beltram]

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PR Submission Checklist for internal contributors

• The PR Title

  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

• The PR Description

  • is free of optional paragraphs and you have filled the relevant parts to the best of your ability

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References

1. https://sparkbox.com/foundry/semantic_commit_messages
2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.

[codecov-commenter]

Codecov Report

Merging #88 (95030b6) into wire/stable (8f9e4a9) will decrease coverage by 0.03%.
The diff coverage is 58.82%.

Additional details and impacted files

@@               Coverage Diff               @@
##           wire/stable      #88      +/-   ##
===============================================
- Coverage        80.86%   80.84%   -0.03%     
===============================================
  Files              173      173              
  Lines            17252    17260       +8     
===============================================
+ Hits             13951    13954       +3     
- Misses            3301     3306       +5     

[OtaK]

Need to make sure the equality comparison is constant-time