Declare a Feature trait, re-implement FeatureProxy

[jgsogo]

• A new Feature trait that is implemented by our current Feature and FeatureProxy.

• Rename our current Feature to FeatureSnapshot, to make it clear it will always return the same evaluation

• Reimplement FeatureProxy making it much straightforward

  pub struct FeatureProxy<'a> {
      client: &'a AppConfigurationClient,
      feature_id: String,
  }

  with this approach, every method can be implemented in terms of our current Feature: we just return a regular feature (with the configuration at that time) and call the method on it.

      fn get_value(&self, entity: &impl Entity) -> crate::errors::Result<super::value::Value> {
          self.client.get_feature(&self.feature_id)?.get_value(entity)
      }

[jgsogo]

This makes it evident that the feature proxy can only live as long as the client is around. See the getter:

pub fn get_feature_proxy<'a>(&'a self, feature_id: &str) -> Result<FeatureProxy<'a>>

This implementation will also let us simplify the internals of the AppConfigurationClient... I'm not 100% sure if this will be the final API, but as long as we don't have a user requesting something different I would choose whatever is easier to implement/maintain.

[rainerschoe]

I like the reference here :-) How would a user use this though? is the proxy still useful?

[rainerschoe]

as discussed: probably like this:

{
let client = AppConfigClient::new(...);
let feature1 = client.get_feature_proxy(...);
let feature2 = client.get_feature_proxy(...);
some_sub_component_entry_point(feature1);
some_other_sub_component(feature2);
}

[jgsogo]

I added these methods just because they are being used in some test or example, but I really think some of them don't belong to the API (or at least, I think we should not expose the models). This is something to tackle in #7

[rainerschoe]

do you think we should change the function name to get_feature_snapshot too?

[jgsogo]

I don't think so, but I'm not sure. Maybe we should change FeatureSnapshot instead. Something like FeatureImpl or similar

[rainerschoe]

No, I like the type name "FeatureSnapshot" :)