Add delegate to simplify AddDocumentStartJavaScriptPlugin implementation

[CrisBarreiro]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1202552961248957/task/1211427307407593?focus=true

Description

• Implement API proposal to simplify using AddDocumentStartJavaScript
• Add AI rules for doc-bot

Steps to test this PR

Pre-requisites

• Apply patch
• Fresh install app
• Filter logs by "Cris"

Check script injected correctly

• Load https://privacy-test-pages.site/privacy-protections/gpc/
• Click "Start test"
• Click details dropdown and check
  • top frame header - "1"
  • top frame JS API - true
  • frame JS API - true

Check messaging works as expected

• Load a site
• Check logs for a message with "featureName":"messaging","context":"contentScopeScripts"
• Open the context menu
• Check logs for {"context":"contentScopeScripts","featureName":"breakageReporting","params":{},"subscriptionName":"getBreakageReportValues"}

Check plugin created only once per tab

• Create a new tab
• Check logs for Creating plugin
• Load a site
• Check no new logs for Creating plugin
• Open tab switcher
• Go back to the same tab
• Check no new logs for Creating plugin
• Create a new tab
• Check there's a new log for Creating plugin

UI changes

n/a

[CrisBarreiro]

• Add delegate to simplify creating implementations of WebMessagingPlugin #6830
• Add delegate to simplify AddDocumentStartJavaScriptPlugin implementation #6827 👈 (View in Graphite)
• develop

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