feat: Add requester as an argument to the third party callback check_event_allowed

[jason-famedly]

Backwards compatibility is maintained for previous versions of the callback that did not have this third argument, which allows for graceful migration when the module it is intended for is updated.

Docs updated to reflect new behavior

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (master@79491c4). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...api/callbacks/third_party_event_rules_callbacks.py	84.61%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #276   +/-   ##
=========================================
  Coverage          ?   80.44%           
=========================================
  Files             ?      502           
  Lines             ?    72416           
  Branches          ?    10911           
=========================================
  Hits              ?    58257           
  Misses            ?    10871           
  Partials          ?     3288           

Files with missing lines	Coverage Δ
synapse/handlers/federation.py	49.60% <ø> (ø)
synapse/handlers/message.py	79.89% <ø> (ø)
...api/callbacks/third_party_event_rules_callbacks.py	68.51% <84.61%> (ø)

---

Continue to review full report in Codecov by Harness.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 79491c4...131b13d. Read the comment docs.

🚀 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.

[nico-famedly]

Commit description should say "Add requester as an argument to...". "New argument" doesn't say much.