[1.x] Externalize KeyPackageStorage and update key package generation API

[tomleavy]

Background:

Part of #211

Key Package Generation API

When creating a key package there are several options that can be set:

• LeafNode extensions
• KeyPackage extensions
• NotBefore time
• NotAfter time

These options should be configured via a new KeyPackageBuilder instead of the current combination of client configuration + properties passed directly to the generate_key_package_message function

Before (0.x)

Creating Key Packages

// Make a key package store that conforms to the KeyPackageStorage trait
let key_package_store = MyKeyPackageStore::new();

let client = Client::builder() 
        .key_package_repo(key_package_store) // Transfer the ownership of the key package repo to a client via the ClientBuilder
        .key_package_lifetime(1000) // All generated key packages are valid for 1000 seconds
        ....
        .build();

// Generate the key package and provide extensions as Default
let key_package = client.generate_key_package_message(Default::default(), Default::default()).unwrap();

After (1.x)

Creating Key Packages

// MyKeyPackageStore no longer needs to conform to KeyPackageStorage (trait no longer exists)
let key_package_store = MyKeyPackageStore::new();

let client = Client::builder()
        .... // No key package specific configuration 
        .build();
        
// Generate a key package and private data
let (kp_message, private_kp_data) = client.key_package_builder()
        .valid_for_sec(1000) // Key package is valid for 1000 seconds
        .with_package_extension(ext1) // Add first extension
        .with_package_extension(ext2) // Add second extension
        .with_leaf_extension(some_leaf_extension) // Add leaf node extension
        .build()
        .unwrap();
        
// Independently store the generated secret keys in MyKeyPackageStore
key_package_store.write(private_kp_data).unwrap();