Feature request: add functionality to retrieve the the server certificate in a client request

## Rationale
This request is related to this [SO Question](https://stackoverflow.com/questions/49154697/aiohttp-how-do-i-retrieve-peer-certificate/49166837#49166837) where I asked whether or not it is possible to get the peer certificate when running a client request.

It is quite logical that the implemented hack is not going to be supported. I do, however, have use-cases in which it is required to retrieve the peer certificate. And I was hoping on working together to find a clean solution.

In protocols like Kerberos and NTLM the server certificate is used in followup steps of the authentication protocol.

## A possible solution

If there is a better solution that would be great as well. Just trying to offer something workable

An extra parameter like `retrieve_peer_certificate` to the method `aiohttp.Client._request` which is default `False` and would make sure that a property like `peer_certificate` would be set in the response object. It would it be possible internally use a `_get_transport` that would return the SSLSocket if the connection is SSL/TLS on which you would be able to call `getpeercert`