[AdminClient] Allow AdminClient to be authenticated

[jerive]

We realised that we could not use the AdminClient in an authenticated fashion.
This PR should address that.

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[jerive]

[clabot:check] El vie, 7 may 2021 a las 9:25, Confluent CLA Bot ***@***.***>) escribió:

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[mhowlett]

thanks @jerive - seems like an oversight

[mhowlett]

this is low risk, so including in v1.7.0