AdminClient DescribeTopicsAsync is impossible to mock

[DainiusCerniauskasAngstrom]

Not sure who decided to make this an extension method rather than put it on the IAdminClient interface, but this makes it impossible to mock, along with the 2 other extension methods.
We cannot even re-write these locally because AdminClient type is internal.

This essentially means any code that uses this is impossible to unit test.