Added custom authentication support to Garnet.

[Xizt]

Changes to support custom authentication. Extended AuthenticationSettings by adding a custom mode which is expected to be passed during server start if custom auth mode is chosen.

[Xizt]

@microsoft-github-policy-service agree company="microsoft"

[vazois]

Need some changes before merging

[Xizt]

@badrishc @vazois There are more dependencies on the GarnetAclAuthenticator. Please do let me know the possibilities of making this class public.
One example, In RespServerSession, we have a check if the authenticator is GarnetAclAuthenticator, then user is set from aclAuthenticator otherwise set to default user. This breaks the session.
Other options I'm evaluating is exposing IGarnetAclAuthenticator interface or instead of passing entire authenticator, passing customValidator.