Add span payload models

[fractalwrench]

Goal

Adds model classes for the new span payloads. This is mostly generated via OpenAPI but tweaked manually for style reasons.

Testing

Added unit test to confirm new model can be constructed from old one.

[priettt]

nit: this is now called data right?

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 79.42%. Comparing base (a7b954f) to head (5c198c4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #520      +/-   ##
==========================================
+ Coverage   79.41%   79.42%   +0.01%     
==========================================
  Files         395      399       +4     
  Lines       10795    10835      +40     
  Branches     1599     1600       +1     
==========================================
+ Hits         8573     8606      +33     
- Misses       1578     1583       +5     
- Partials      644      646       +2     

Files	Coverage Δ
...e/android/embracesdk/internal/payload/Attribute.kt	100.00% <100.00%> (ø)
...mbrace/android/embracesdk/internal/payload/Span.kt	100.00% <100.00%> (ø)
...e/android/embracesdk/internal/payload/SpanEvent.kt	100.00% <100.00%> (ø)
.../android/embracesdk/internal/payload/SpanMapper.kt	78.94% <78.94%> (ø)

... and 2 files with indirect coverage changes