Use userName instead of fullName in AuthenticationResult for KerberosAuthenticator

[JWuCines]

Description

AuthenticationResult can be used for Authorization purpose.

AuthenticationResult includes all the metadata after authentication process with the authentication authority.
In case of Kerberos, it returns the fullName, which includes the username and the realm.
However, for authorization purpose, most of the time only the username is required (i.e. Ranger).

Release note

AuthenticationResult from KerberosAuthenticator can be used for Authorization (i.e. Ranger)

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Key changed/added classes in this PR

• AuthenticationResult

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This PR has:

• been self-reviewed.
• a release note entry in the PR description.
• been tested in a Druid cluster.

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