Feature Flag evaluation event

[dyladan]

First draft of feature_flag.evaluation event from the feature flag semantic conventions working group. Based on proposal here: https://docs.google.com/document/d/1sBDD-uifx-Qy0NxtwRcrRAQ48sHQS9dTp6tpArQsoZg/edit

Summary of changes:

• Replace feature_flag.provider_name with feature_flag.provider.id feature_flag.system
• Rename feature_flag to feature_flag.evaluation
• Add attributes to feature_flag.evaluation
  • feature_flag.context.id
  • feature_flag.environment.id feature_flag.flag_set.id
  • feature_flag.value
  • feature_flag.version
• Remove feature flag span event
• Add feature_flag.error.code with values from https://openfeature.dev/specification/types/#resolution-details
• Add feature_flag.reason with values from https://openfeature.dev/specification/types/#resolution-details

[askpt]

I added a couple of comments; otherwise, it looks good.

[toddbaert]

I left some small nits (perhaps not in the right places; I'm not sure if some portion of this is generated or not) but approving.