Use event.name as attribute key for exporting Log event name

[cijothomas]

This is still being discussed in Event SIG and semantic conventions, but I propose to name it "event.name" before the upcoming release. There is high chance that "event.name" will be the final name, so we can avoid yet another breaking change after the coming release.
This is still under feature-flag.

(Similar change for stdout exporter is needed, but that'll be done the current refactoring of stdout exporter to avoid conflicts)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.3%. Comparing base (b24bb9c) to head (9ec7986).
Report is 1 commits behind head on main.

Files	Patch %	Lines
opentelemetry-proto/src/transform/logs.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2050     +/-   ##
=======================================
- Coverage   77.3%   77.3%   -0.1%     
=======================================
  Files        124     124             
  Lines      21282   21282             
=======================================
- Hits       16472   16471      -1     
- Misses      4810    4811      +1     

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