Add put(AttributeKey<T>, T) overload to EventBuilder

[jack-berg]

Event payload fields seem to be different than attributes: open-telemetry/semantic-conventions#505

That's expected to some degree, since attribute types are limited to primitives and arrays of primitives, and event payloads are intended to be able to encode complex structured data, like bytes, object arrays, and maps of maps. But excluding Attributes / AttributesKey<?> from the event payload API surface area seems odd, given how prevalent Attributes are in the other APIs.

What happens if I want to include an attribute generated from semantic-conventions-java in my payload? Am I prohibited from doing that? Do I need to call put(AttributeKey.getKey(), value) to extract the string attribute key and loose type safety? This is poor ergonomics.

I'm not exactly sure where things will land with how attributes and event fields overlap (if at all), but it seems like we ought to support our notion of AttributeKey in the API regardless. This PR adds that surface area. We might also consider adding a putAll(Attributes) convenience mechanism for adding all the key value from a bundle of attributes into the payload.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 91.07%. Comparing base (98eded9) to head (e19cabb).

Files	Patch %	Lines
...entelemetry/api/incubator/events/EventBuilder.java	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6331      +/-   ##
============================================
- Coverage     91.07%   91.07%   -0.01%     
- Complexity     5751     5759       +8     
============================================
  Files           626      626              
  Lines         16782    16800      +18     
  Branches       1718     1718              
============================================
+ Hits          15285    15300      +15     
- Misses         1003     1004       +1     
- Partials        494      496       +2     

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[jkwatson]

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