event logger spec compliance

[brettmc]

update event logger to conform to spec 1.34.0, specifically:

• implement ForceFlush, which delegates to the LoggerProvider
• rename payload to body

Fixes: #1328
Fixes: #1327

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.43%. Comparing base (eebf23a) to head (011e901).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1335      +/-   ##
============================================
+ Coverage     74.29%   74.43%   +0.13%     
- Complexity     2491     2494       +3     
============================================
  Files           353      354       +1     
  Lines          7135     7142       +7     
============================================
+ Hits           5301     5316      +15     
+ Misses         1834     1826       -8     

Flag	Coverage Δ
8.1	73.99% <90.00%> (-0.05%)	⬇️
8.2	74.33% <90.00%> (+0.16%)	⬆️
8.3	74.18% <90.00%> (-0.08%)	⬇️
8.4	74.25% <90.00%> (+0.02%)	⬆️

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Files	Coverage Δ
src/SDK/Logs/EventLogger.php	100.00% <100.00%> (ø)
src/SDK/Logs/EventLoggerProvider.php	100.00% <100.00%> (ø)
src/SDK/Logs/EventLoggerProviderFactory.php	100.00% <100.00%> (ø)
src/SDK/Logs/NoopEventLoggerProvider.php	100.00% <100.00%> (ø)
src/API/Logs/NoopEventLogger.php	0.00% <0.00%> (ø)

... and 5 files with indirect coverage changes

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