feat(mq): improve usability of key expressions

[savonarola]

Fixes EMQX-14739

Release version: 6.0.0

Summary

PR Checklist

• For internal contributor: there is a jira ticket to track this change
• The changes are covered with new or existing tests
• Change log for changes visible by users has been added to changes/ee/(feat|perf|fix|breaking)-<PR-id>.en.md files
• Schema changes are backward compatible or intentionally breaking (describe the changes and the reasoning in the summary)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.66%. Comparing base (c3d4722) to head (eb44a3a).
⚠️ Report is 7 commits behind head on release-60.

Additional details and impacted files

@@              Coverage Diff               @@
##           release-60   #16041      +/-   ##
==============================================
- Coverage       84.71%   84.66%   -0.06%     
==============================================
  Files            1150     1150              
  Lines           79378    79384       +6     
==============================================
- Hits            67248    67209      -39     
- Misses          12130    12175      +45     

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[emqxqa]

TestExecution