Implement builder pattern for AppConfigurationClient

[jgsogo]

closes #31

Now that I've written this, I wonder if it would be easier to have a AppConfigurationClient trait and multiple implementations:

AppConfigurationIBMCloud::new(region, api_key);

AppConfigurationLocal::new(filepath);

AppConfigurationURLServer::new(url);

[jgsogo]

pub trait AppConfigurationClient {  
   fn get_feature(&self, feature_id: &str) -> Result<Feature>;
}

// Connecting to IBM Cloud to get the snapshots (and keep them updated)
pub struct AppConfigurationIBMCloud {}

impl AppConfigurationIBMCloud {
   pub fn new(...) -> Self {}
}

impl AppConfigurationClient for AppConfigurationIBMCloud { 
    fn get_feature(&self, feature_id: &str) -> Result<Feature> {
        todo!("impl")
    }
};

// Reading the snapshot from a file
pub struct AppConfigurationFileSnapshot {}

impl AppConfigurationFileSnapshot {
   pub fn new(...) -> Self {}
}

impl AppConfigurationClient for AppConfigurationFileSnapshot { 
    fn get_feature(&self, feature_id: &str) -> Result<Feature> {
        todo!("impl")
    }
};

[jgsogo]

superseded by #34