Add createdAt time for signInWithIdp new users

[lisajian]

Description

As the title suggests. Should fix #5203.

Scenarios Tested

• npm test passes
• Some manual verification (since unit testing this is a little difficult w/o time mocks aside from checking that the createdAt time is present):
  • go/firebase-tools-5260-prod: Behavior in prod, timestamp for lastSignInTime matches that of createdAt
  • go/firebase-tools-5260-emulator: Behavior against emulator and test project with this fix, timestamp for lastSignInTime matches that of createdAt

Sample Commands

N/A

[codecov-commenter]

Codecov Report

Base: 56.27% // Head: 56.27% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (f3c2973) compared to base (b2d23b6).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5260   +/-   ##
=======================================
  Coverage   56.27%   56.27%           
=======================================
  Files         309      309           
  Lines       20819    20820    +1     
  Branches     4224     4224           
=======================================
+ Hits        11715    11716    +1     
  Misses       8092     8092           
  Partials     1012     1012           

Impacted Files	Coverage Δ
src/emulator/auth/operations.ts	83.41% <100.00%> (+0.01%)	⬆️

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