add TuneAuth() function to Vault client package

[infa-bsurber]

Is your feature request related to a problem? Please describe.
Right now I can access mount tuning via the TuneMount function, which is hardcoded to use sys/mounts/:path/tune, but there is no option to tune auth backends.

Describe the solution you'd like
Add a function to enable tuning auth backends called TuneAuth that makes use of this API endpoint: https://www.vaultproject.io/api/system/auth.html#tune-auth-method

Describe alternatives you've considered
Make tuning a more generic function (called Tune) that can handle Mounts and Auths both if the function signatures are similar enough

Explain any additional use-cases

Additional context
Right now this has to be done via the Logical() functions instead