test: add a test as an example of using did-comm mediation

[mirceanis]

What is being changed

This adds a test that shows how to use the did-comm mediation features.

Quality

Check all that apply:

• I want these changes to be integrated
• I successfully ran pnpm i, pnpm build, pnpm test, pnpm test:browser locally.
• I allow my PR to be updated by the reviewers (to speed up the review process).
• I added unit tests.
• I added integration tests.

Details

The mediation flow is quite cumbersome and strict. Proposals welcome for improving the state of things.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 65.62500% with 11 lines in your changes missing coverage. Please review.

Project coverage is 89.95%. Comparing base (c384fb9) to head (aec9fcd).
Report is 19 commits behind head on next.

Files	Patch %	Lines
...d-comm/src/protocols/trust-ping-message-handler.ts	52.63%	9 Missing ⚠️
packages/remote-server/src/messaging-router.ts	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #1355      +/-   ##
==========================================
+ Coverage   89.82%   89.95%   +0.12%     
==========================================
  Files         176      176              
  Lines       27678    27695      +17     
  Branches     2180     2200      +20     
==========================================
+ Hits        24861    24912      +51     
+ Misses       2817     2783      -34     

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