Optimize xpdata/context for persistent queue when only one value for key

[bogdandrutu]

This is a backwards compatible change because on the decoding side we support both single value string as well as slice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.77%. Comparing base (336f54c) to head (03b5c48).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13636      +/-   ##
==========================================
- Coverage   92.78%   92.77%   -0.01%     
==========================================
  Files         630      630              
  Lines       35511    35521      +10     
==========================================
+ Hits        32949    32955       +6     
- Misses       2018     2021       +3     
- Partials      544      545       +1     

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[dmitryax]

LGTM