feat: Added date to order invoice

[mrsaicharan1]

Fixes #6312

Short description of what this resolves:

Adds the date the order invoice was generated

Changes proposed in this pull request:

• created_at is added to invoice

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

[codecov]

Codecov Report

Merging #6313 into development will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development    #6313   +/-   ##
============================================
  Coverage        65.32%   65.32%           
============================================
  Files              287      287           
  Lines            14695    14695           
============================================
  Hits              9600     9600           
  Misses            5095     5095

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