feat: add time provider package

[Anaswara-Suresh]

Summary by cubic

Add a built-in time provider for CUEX to convert between dates and UNIX timestamps with optional layouts, and register it in the default internal packages. This supports GWCP-94846 by expanding built-in packages used in testing.

• New Features
  • Added providers/time with two ops: "timestamp" (date → int64) and "date" (int64 → string), defaulting to RFC3339 and UTC.
  • Exposed CUE templates: #DateToTimestamp and #TimestampToDate with optional layout params.
  • Registered timeext.Package in the compiler’s default internal packages.
  • Added unit tests covering RFC3339/RFC1123, empty inputs, and error cases.

^{Written for commit e1202e3. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.33%. Comparing base (ef68242) to head (e1202e3).

Files with missing lines	Patch %	Lines
cue/cuex/providers/time/time.go	81.25%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #127      +/-   ##
==========================================
- Coverage   88.34%   88.33%   -0.01%     
==========================================
  Files          82       83       +1     
  Lines        3491     3524      +33     
==========================================
+ Hits         3084     3113      +29     
- Misses        273      278       +5     
+ Partials      134      133       -1     

Flag	Coverage Δ
unit-test	88.33% <81.81%> (-0.01%)	⬇️

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