Add support for Italian DGC settings

[dodomorandi]

This is a prerequisite for #38.

Support for the school parameters is still missing -- there are only end fields, start is missing. Probably these fields must be related to some other start field, probably it will be more clear once we start implementing proper validation.

I am also not sure about IncompleteSettings: should it be non_exhaustive?

[codecov-commenter]

Codecov Report

Merging #39 (bcefef3) into main (aee00bd) will increase coverage by 0.39%.
The diff coverage is 85.58%.

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
+ Coverage   84.73%   85.12%   +0.39%     
==========================================
  Files          11       12       +1     
  Lines         989     1862     +873     
==========================================
+ Hits          838     1585     +747     
- Misses        151      277     +126     

Impacted Files	Coverage Δ
dgc-italy-core/src/settings.rs	85.56% <85.56%> (ø)
dgc-italy-core/src/lib.rs	100.00% <100.00%> (ø)

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[lu-zero]

Land it, we can iterate between validation and parsing from this PR.