Support persistent caching

[rainerschoe]

Provide file path to use as persistent cache.
The client then should read this cache at startup and use it as initial values until server connection is established.
The file should be written on updates from the server to keep it up to date

[rainerschoe]

Javi and me just discussed this and came up with a brilliant idea:

We can extend the OfflineMode like this:

#[derive(Debug)]
pub enum OfflineMode {
    /// Returns errors when requesting features or evaluating them
    Fail,

    /// Return features and values from the latests configuration available
    Cache(Option<PathBuf>),

    /// Use the provided configuration.
    FallbackData(AppConfigurationOffline),
}

Then the user can add a optional filepath to the Cache which would mean:

• Cache is on startup populated with this file if it exists
• Whenever updates from server arrive, the file is updated

This would then also apply in the initializing stage.

How would this work together with #67 ?

• We will only provide a wait_until_online function. The user can then determine on his own if he wants to call it:
• If he provides a existing cache file and selects OfflineMode::Cache(Some(...)), he will not need to wait
• Otherwise he will need to wait