chore: Move stringDecode() to CommonStringExprs trait

[peter-toth]

Which issue does this PR close?

Follow-up to #2075.

Rationale for this change

See discussion #2075 (comment).

What changes are included in this PR?

Introduce CommonStringExprs and move stringDecode() to there.

How are these changes tested?

Existing tests.

[mbutrovich]

Thanks for the followup PR, @peter-toth!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.19%. Comparing base (f09f8af) to head (ce3a782).
⚠️ Report is 1164 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/main/scala/org/apache/comet/serde/strings.scala	66.66%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2111      +/-   ##
============================================
+ Coverage     56.12%   58.19%   +2.06%     
- Complexity      976     1252     +276     
============================================
  Files           119      141      +22     
  Lines         11743    13204    +1461     
  Branches       2251     2360     +109     
============================================
+ Hits           6591     7684    +1093     
- Misses         4012     4289     +277     
- Partials       1140     1231      +91     

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[peter-toth]

Thanks for the review @mbutrovich !