Add value_type to feature flag event

[dyladan]

This adds a value_type which references the body field value in feature flag evaluation events. The value field has an undefined type, allowing any type to be included. This means processing pipelines have to sniff the type before applying any logic. In some cases this may even result in ambiguous cases. For example if a processing pipeline stringifies the value, it may be impossible to tell the difference between boolean false and string "false".

[toddbaert]

Non-binding approval from me ✔️