Switchable Nonce Handling Strategy for HTTP DigestAuthenticator

Allow the nonce strategy to be switchable: -

1 - Real 'Number Used Once' - i.e. new nonce for each request.
2 - Nonce per connection i.e. as long as a connection is kept alive allow re-use of nonce - new nonce on new connection.
3 - Timed nonce - Generate a nonce with a server secret and timestamp, nonce will be accepted for a validity period.