Change certain settings per request

[rudolphfroger-ds]

It would be nice to change certain things per request which are currently set on the Client. For some settings the Client acts as a way to set default settings. My use case would be setting a different client certificate depending on the hostname. One can work around this by using multiple Clients but it would be nice to keep the connection pooling and other advantages of sharing a Client between requests. Setting default_headers already works this way.

I'm not sure if this is worth the trouble. Maybe reqwest users should just keep a Client per hostname (for example) if they want different connection settings per hostname.

Also see #353.

[seanmonstar]

In general terms, it's probably a good idea to allow request options to override client options. I think timeout already works that way... it'd probably be better to come up with a system that applies all options, instead of one-off each option...

[seanmonstar]

But also, some settings fundamentally can't be changed per request, since the are options on shared types like the connector or pool.