refactor(*) generalize ngx_str_node_t and ngx_str_t extensions

[thibaultcha]

To use them in IPC code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.74605%. Comparing base (c46ff1c) to head (cfcd057).
Report is 1 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main        #606         +/-   ##
===================================================
+ Coverage   90.73673%   90.74605%   +0.00932%     
===================================================
  Files             52          52                 
  Lines          11076       11152         +76     
===================================================
+ Hits           10050       10120         +70     
- Misses          1026        1032          +6     

Files with missing lines	Coverage Δ
src/ngx_wasmx.h	100.00000% <100.00000%> (ø)
src/wasm/vm/ngx_wavm.c	87.32970% <100.00000%> (+0.15729%)	⬆️

... and 26 files with indirect coverage changes

Flag	Coverage Δ
unit	90.48593% <100.00000%> (+0.03869%)	⬆️
valgrind	82.32801% <100.00000%> (-0.08270%)	⬇️

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