Remove `FeatureContext.feature_key`, `SegmentResult.key`

[khvn26]

After Flagsmith/flagsmith#6176 is merged:

• Remove SegmentResult.key.
• Remove FeatureContext.feature_key. FeatureContext.name should be used for override matching.
• Generate IdentityContext.key (EnvironmentContext.key + "_" + IdentityContext.identifier`) unless it's provided explicitly.