fix(internal): change url_encode include from .h to .cc

[alevenberg]

Part of #12232

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This change is 

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (6e9d901) 93.64% compared to head (2e760b2) 93.63%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12547      +/-   ##
==========================================
- Coverage   93.64%   93.63%   -0.01%     
==========================================
  Files        2044     2044              
  Lines      181278   181278              
==========================================
- Hits       169750   169744       -6     
- Misses      11528    11534       +6     

Files Changed	Coverage Δ
...internal/golden_kitchen_sink_metadata_decorator.cc	84.31% <ø> (ø)
.../internal/golden_kitchen_sink_metadata_decorator.h	100.00% <ø> (ø)
.../internal/golden_thing_admin_metadata_decorator.cc	92.02% <ø> (ø)
...1/internal/golden_thing_admin_metadata_decorator.h	100.00% <ø> (ø)
generator/internal/metadata_decorator_generator.cc	34.54% <100.00%> (ø)

... and 8 files with indirect coverage changes

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