feature: read-only clients

[davepacheco]

It would be very handy to be able to create a reqwest client that could only make "read" requests (e.g., GET or HEAD). I imagine we could use Method::is_safe() for this?

This would allow applications to more safely implement "dry run" or "debug-only" clients that couldn't accidentally do something destructive to the server.

[seanmonstar]

The use case sounds cool!

Though, I'm leaning more towards an implementation not being something directly in reqwest, but rather a Middleware. That combined with my goal of making reqwest integrate with Tower would mean anyone could get that behavior wrapped around any stacked client.