feat: add IPACodesURL option to ParserConfig

[creed-bratton]

Fixes #348.

Adds ParserConfig.IPACodesURL: when set, NewParser fetches a fresh Italian IPA codes list from the given URL instead of using the embedded snapshot. The Agid export URL can be passed directly (one code per line, matches the existing embedded format).

p, err := publiccode.NewParser(publiccode.ParserConfig{
    IPACodesURL: "https://www.indicepa.gov.it/public-services/opendata-read-service.php?dstype=FS&filename=amministrazioni.txt",
})

Leave IPACodesURL empty (default) to keep using the embedded snapshot — no behaviour change for existing callers. WASM callers omit the option and continue to use the snapshot as before.

Implementation notes:

• The validator is now per-Parser rather than a package-level global, so concurrent parsers with different code sets are safe.
• MakeIsOrganisationURI builds its inner validator once per Parser (closure) instead of on every call to isOrganisationURI.
• validators.New now takes the codes map; validators.DefaultIPACodes exposes the embedded set for callers that need it.

[codecov]

Codecov Report

❌ Patch coverage is 90.62500% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.58%. Comparing base (6682066) to head (1707378).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
parser.go	81.25%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #352      +/-   ##
==========================================
- Coverage   84.76%   84.58%   -0.18%     
==========================================
  Files          15       15              
  Lines        1201     1226      +25     
==========================================
+ Hits         1018     1037      +19     
- Misses        155      158       +3     
- Partials       28       31       +3     

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