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 ✔️

[lmolkova]

It's not for this PR, but we're seriously considering allowing complex attribute types instead of body

#1669 open-telemetry/opentelemetry-specification#4334.

Wonder what your thoughts are

[lmolkova]

@dyladan could you please elaborate on

This means processing pipelines have to sniff the type before applying any logic.

the type information is a part of OTLP (e.g. here's a map attribute json example)

{
  "key": "map.attribute",
  "value": {
    "kvlistValue": {  # type info
      "values": [
        {
          "key": "some.map.key",
          "value": {
            "stringValue": "some value"   # type info in stringValue
          }
        }
      ]
    }
  }

body should be represented in the same way. So I wonder why is it a problem for the pipeline - it should know the type, no?