Consider warning caller if supplied client params will in favour of transport

[declension]

First of all, thanks for a great library!

I recently got surprised when debugging something to find that if you do:

import httpx
...

transport = httpx.AsyncHTTPTransport(retries=3)
return httpx.AsyncClient(transport=transport, base_url="https://example.com", trust_env=False, cert=cert)

You won't get cert config trust_env respected (but your base_url will be)

I agree that the precedence logic (transport overriding params) probably makes sense, but it feels risky to take these parameters, some of which set up important security features, and silently ignore them if another parameter is supplied.

It seems some others have been surprised here (See #2888, #2513 (comment))

Note also this might probably come into focus more now, with the deprecation of app=app method of setting up a client (this is how we got to creating a transport manually).

Perhaps a warning of some sort might help here?