Extract the it-sdi addon to an external module

[rilla]

• Extracts the Italian SDI addon (it-sdi-v1) to the standalone github.com/invopop/gobl.it.sdi module; the key stays recognised via core's approved-external-addon list.
• Decouples the remaining core tests (IT regime legacy-key migration, totals calculator) from the addon's ext-key constants; addon-behaviour tests move to the module.

Related: invopop/gobl.fatturapa#71

Pre-Review Checklist

• Opened this PR as a draft
• Read the CONTRIBUTING.md guide.
• Performed a self-review of my code.
• Added thorough tests with at least 90% code coverage.
• Modified or created example GOBL documents to show my changes in use, if appropriate.
• Added links to the source of the changes in tax regimes or addons, either structured or in the comments.
• Run go generate . to ensure that the Schemas and Regime data are up to date.
• Reviewed and fixed all linter warnings.
• Been obsessive with pointer nil checks to avoid panics.
• Updated the CHANGELOG.md with an overview of my changes.
• Marked this PR as ready for review.

And if you are part of the org:

• Requested a review from Copilot and fixed or dismissed (with a reason) all the feedback raised.
• Requested a review from @samlown.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.70%. Comparing base (7a4ad62) to head (8d1a30a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #876      +/-   ##
==========================================
- Coverage   94.73%   94.70%   -0.03%     
==========================================
  Files         316      310       -6     
  Lines       17198    16767     -431     
==========================================
- Hits        16292    15879     -413     
+ Misses        560      553       -7     
+ Partials      346      335      -11     

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[samlown]

Looks good! Conflicts just need fixing.

[rilla]

Rebased and fixed conflicts 👍🏼