Remove callback severance on 5-to-3 adapter

[bretambrose]

1. Strip out everything related to the callback firewall on shutdown
2. Add a vtable to adapter operations and a fail function for publish, subscribe, unsubscribe
3. Invoke fail on all incomplete operations during final adapter shutdown
4. Add a test that verifies completion callback invocation after the adapter is shut down

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (0cc50d1) 82.34% compared to head (4f5f9a9) 82.38%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #329      +/-   ##
==========================================
+ Coverage   82.34%   82.38%   +0.04%     
==========================================
  Files          20       20              
  Lines        8738     8721      -17     
==========================================
- Hits         7195     7185      -10     
+ Misses       1543     1536       -7     

Files	Coverage Δ
source/v5/mqtt5_to_mqtt3_adapter.c	76.33% <92.68%> (+0.18%)	⬆️

... and 2 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.