Add "invocation_started_at" global field

[gshank]

resolves #245

Description

Add new global "invocation_started_at" which is created and reset at the same time as "invocation_id".

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.90%. Comparing base (397fb3f) to head (3d4fbe3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #246      +/-   ##
==========================================
+ Coverage   69.83%   69.90%   +0.07%     
==========================================
  Files          52       52              
  Lines        3517     3522       +5     
==========================================
+ Hits         2456     2462       +6     
+ Misses       1061     1060       -1     

Flag	Coverage Δ
unit	69.90% <100.00%> (+0.07%)	⬆️

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[QMalcolm]

It might be that this dbt-common way is the better way to be doing this. However, we do already provide an invoked_at in the RuntimeConfig. Whether that changes the necessity of this work in dbt-common, I'm not sure

[QMalcolm]

Ship it! We should get dbt-core updated to use this for the setting of invoked_at on the RuntimeConfig.