Add Client.auth setter

[florimondmanca]

Closes #1066

Make Client.auth / AsyncClient.auth properties, so that .auth can be set using the same argument types and logic than passed on client init.

client=httpx.Client()
client.auth= ("username", "p@ssw0rd")
response=client.get(...) # Uses Basic Auth.

Implementation-wise, we split the existing ._build_auth(request, auth) helper into ._build_auth(auth) and ._build_request_auth(request, auth). The former converts AuthTypes to an Optional[Auth] and is used in the .auth setter as well as in ._build_request_auth() to convert the request-level auth info.