Add hasEnded to ReadableSpan and SpanData (split from #693)

[Oberon00]

This PR was split from #693 at the request of @bogdandrutu. It just renames the internal boolean hasBeenEnded of RecordEventsReadableSpan to hasEnded, makes it public on the ReadableSpan interface and also adds it to SpanData.

[codecov-io]

Codecov Report

Merging #733 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #733      +/-   ##
============================================
+ Coverage     78.71%   78.74%   +0.02%     
- Complexity      746      747       +1     
============================================
  Files            93       93              
  Lines          2608     2611       +3     
  Branches        235      235              
============================================
+ Hits           2053     2056       +3     
  Misses          459      459              
  Partials         96       96

Impacted Files	Coverage Δ	Complexity Δ
...main/java/io/opentelemetry/sdk/trace/SpanData.java	100% <ø> (ø)	2 <0> (ø)	⬇️
...ntelemetry/sdk/trace/RecordEventsReadableSpan.java	89.47% <100%> (+0.18%)	49 <3> (+1)	⬆️

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