`HarborAsyncClient` is too verbose of a name

[pederhan]

The name HarborAsyncClient is technically correct, but is incredibly verbose. As this package is async-first, having Async in the class name seems superfluous. While we can't get rid of this name entirely because of backwards-compatibility, we can introduce a new alias that can be imported instead.

Some suggestions:

• Harbor
• HarborAPI
• HarborClient
  • In this case we need to break backwards-compatibility with imports of harbor.HarborClient, which currently is a shorthand for harbor.client_sync.HarborClient, and then rename the existing one to harbor.client_sync.HarborSyncClient.