(change) Allow empty evaluation context for some flags.

[thomaspoignant]

Motivation

As of today all flag evaluation are needing at least a targetingKey.
The reason behind that is because the targetingKey is used to do bucketing during the flag evaluation (specially for percentage based rules)

But not all flags requires bucketing, so in some cases it should be ok, not to have a bucketing key (targetingKey or any other bucketingKey).

Example

my-new-feature:
  variations:
    enabled: true
    disabled: false
  defaultRule:
    variation: disabled

A flag like this one does not require a bucketing key, so we should accept an empty evaluation context for such flag.

Requirements

• We should allow empty evaluation context for flag that does not require it.
• In case of the flag requires a bucketing key and has no targetingKey we should not error if the flag is configured to use a different bucketingKey.
• In case of the flag use a percentage based rollout and that we don't have any key to do the variant affection we should error with TARGETING_KEY_MISSING.
• Today all OF providers are checking the presence of the targetingKey, we should remove this mandatory thing and let the evaluation goes to the relay-proxy before returning any errors.

This change is quite impactful and touch a lot of core components of GO Feature Flag, if you want to work on it please reach out before jumping on the implementation.

[thomaspoignant]

A way to address this is to check if the flag need a bucketing key.
Bucketing keys are useful for percentage and progressive rollout, so a function like this one can tell if we need a bucketing key or not:

func (f *InternalFlag) flagNeedBucketingKey() bool {
	if f.DefaultRule.Percentages != nil {
		return true
	}
	if f.DefaultRule.ProgressiveRollout != nil {
		return true
	}

	for _, rule := range f.GetRules() {
		if rule.Disable != nil && *rule.Disable {
			continue
		}
		if rule.Percentages != nil && len(rule.GetPercentages()) > 0 {
			return true
		}
		if rule.ProgressiveRollout != nil {
			return true
		}
	}
	
	return false
}