refactor(*) generalize bytes readers for all ngx_wa

[thibaultcha]

These will be used in IPC code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.71783%. Comparing base (80da044) to head (d5235b0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main        #597         +/-   ##
===================================================
- Coverage   90.72686%   90.71783%   -0.00903%     
===================================================
  Files             51          52          +1     
  Lines          11075       11075                 
===================================================
- Hits           10048       10047          -1     
- Misses          1027        1028          +1     

Files with missing lines	Coverage Δ
src/common/ngx_wa_readers.c	100.00000% <100.00000%> (ø)
src/common/ngx_wasm_socket_tcp_readers.c	87.61062% <100.00000%> (-0.20451%)	⬇️

... and 2 files with indirect coverage changes

Flag	Coverage Δ
unit	90.44625% <100.00000%> (ø)
valgrind	82.28991% <100.00000%> (-0.06945%)	⬇️

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