perf: GetField reduce allocations

[M4tteoP]

One less alloc/op, but numbers can matter at scale.

Before:

goos: darwin
goarch: arm64
pkg: github.com/corazawaf/coraza/v3/internal/corazawaf
cpu: Apple M1 Pro
BenchmarkTxGetField-10    	 2294287	       582.5 ns/op	     448 B/op	       9 allocs/op
PASS
ok  	github.com/corazawaf/coraza/v3/internal/corazawaf	2.374s

BenchmarkCRSSimplePOST-10    	     902	   1276337 ns/op	  338771 B/op	    7220 allocs/op
PASS
ok  	github.com/corazawaf/coraza/v3/testing/coreruleset	2.054s

After:

goos: darwin
goarch: arm64
pkg: github.com/corazawaf/coraza/v3/internal/corazawaf
cpu: Apple M1 Pro
BenchmarkTxGetField-10    	 3306369	       355.6 ns/op	     400 B/op	       8 allocs/op
PASS
ok  	github.com/corazawaf/coraza/v3/internal/corazawaf	2.051s

BenchmarkCRSSimplePOST-10    	     874	   1262635 ns/op	  320807 B/op	    6418 allocs/op
PASS
ok  	github.com/corazawaf/coraza/v3/testing/coreruleset	2.034s

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.46%. Comparing base (4ff1f76) to head (f0e9fec).
Report is 115 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1195      +/-   ##
==========================================
+ Coverage   82.72%   83.46%   +0.74%     
==========================================
  Files         162      166       +4     
  Lines        9080     9611     +531     
==========================================
+ Hits         7511     8022     +511     
- Misses       1319     1338      +19     
- Partials      250      251       +1     

Flag	Coverage Δ
default	83.46% <100.00%> (+5.62%)	⬆️
examples	83.46% <100.00%> (+57.03%)	⬆️
ftw	83.46% <100.00%> (+36.09%)	⬆️
ftw-multiphase	83.46% <100.00%> (+33.92%)	⬆️
tinygo	83.46% <100.00%> (+8.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.