feat(async/unstable): add Lazy for once-only async init

[tomas-zijdemans]

A common pattern is having a value that is expensive to compute, should only be computed once, and should be shared across concurrent callers. Think: database connections or config loaded from disk (or fetched from remote).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.17%. Comparing base (6ab920d) to head (c6ddad5).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7007      +/-   ##
==========================================
- Coverage   94.20%   94.17%   -0.03%     
==========================================
  Files         618      619       +1     
  Lines       48802    48843      +41     
  Branches     8601     8605       +4     
==========================================
+ Hits        45973    45997      +24     
- Misses       2759     2777      +18     
+ Partials       70       69       -1     

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[kt3k]

LGTM