The SCRAM RFC describes extensions that can be used to add extra
data into the protocol.
This commit adds support for the client scram mechanism to insert extension
data into the client messages at the locations specified by the SCRAM RFC.
Kafka utilizes these extensions when authenticating delegation tokens
over scram. Since I am writing a kafka client I would like access to
these extensions so I can support delegation tokens.
I've only added them to Scram::new not Mechanism::from_credentials
since they do not apply to other mechanisms. For my purposes this is
fine since I only need to work with scram. However it would be
limiting for other use cases, so I'm quite happy to add the extension
fields into Credentials if that was desired. For now I've left it out
since the fields would be scram exclusive and everything else in
Credentials is currently generic.
