[security] Implement SASL username/password auth (prerequisite for JWT authentication)

[eolivelli]

Motivation

• In order to implement JWT token authentication we need a auth mechanism that allows you to pass a plain token
• The idea will be eventually to carry the JWT token in the "password" field (like we do it in https://github.com/datastax/startlight-for-kafka)

Modification

• implemented the PLAIN auth mechanism on both the client and the server
• add test cases
• implement a DigestModule in order to not require people use ZooKeeper implementation

Note

Please note that MD5-DIGEST is recently considered not secure, adding a username/password mechanism doesn't add a security issue.

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