BunnyCDNStorage make properties {init;} instead of "private"

[benrussill]

public string ApiAccessKey { get; private set; }

• Change private to init
• also, why is it public? Can you remove the get

public string StorageZoneName { get; private set; }

• Change private to init
• also, why is it public? Can you remove the get

[OhSoooLucky]

This is done for backwards compatibility with netstandard which do not support the init yet.
I've added a compile flag to handle this for non-standard builds