Implement LeapSecondsProvider trait with builtin and LSK impls

[helgee]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 98.90710% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 98.10%. Comparing base (7937a99) to head (600cfc5).
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/lox-io/src/spice.rs	98.46%	1 Missing ⚠️
crates/lox-time/src/utc/leap_seconds.rs	98.79%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
==========================================
+ Coverage   98.08%   98.10%   +0.01%     
==========================================
  Files          54       55       +1     
  Lines       19189    19325     +136     
==========================================
+ Hits        18822    18958     +136     
  Misses        367      367              

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