Leverage od_start_template_optimization action to initialize common LCP external background image data

[westonruter]

This depends on #1919

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.73%. Comparing base (6abee9c) to head (d6433ba).

Additional details and impacted files

@@                       Coverage Diff                        @@
##           update/od-initialization-timing    #1921   +/-   ##
================================================================
  Coverage                            70.73%   70.73%           
================================================================
  Files                                   86       86           
  Lines                                 7001     7001           
================================================================
  Hits                                  4952     4952           
  Misses                                2049     2049           

Flag	Coverage Δ
multisite	70.73% <100.00%> (ø)
single	40.67% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.