Add createdOn time to context

[dfarr]

This time indicates the time that the durable promise was created, if an execution has opted out of a durable promise the time that the invocation was created will be used instead.

[dfarr]

What is your preferred term?

• createdOn (what we use in the server/db)
• createdAt
• something else?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 81.17%. Comparing base (18fe922) to head (176e288).

Files	Patch %	Lines
lib/async.ts	0.00%	2 Missing ⚠️
lib/generator.ts	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #107      +/-   ##
==========================================
- Coverage   81.32%   81.17%   -0.15%     
==========================================
  Files          22       22              
  Lines        1296     1307      +11     
  Branches      348      327      -21     
==========================================
+ Hits         1054     1061       +7     
- Misses        234      246      +12     
+ Partials        8        0       -8     

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