[Tokio migration] Proxy support

[Johannesd3]

Proxy support was one of the few things that weren't easy to migrate to tokio 1.0, mainly because hyper-proxy is not migrated yet. While #583 added basic proxy support again, it was only tested with a quickly set up local proxy server instance. In the discussion of this PR, 3 alternatives came up (read there for their advantages an drawbacks):

1. Using the simple proxytunnel
2. Waiting for hyper-proxy
3. Using reqwest

Since I don't know anything about proxy servers (and don't need them), I would appreciate if someone else takes a look at it.

@sashahilton00 Would it be possible to use milestones or custom labels to keep track of the issues related to the tokio migration?

[Johannesd3]

They did update hyper-proxy, so we can make progress here.

BTW, while re-implementing proxy support I did not really care for the format of addresses (e.g. whether port or protocol are specified), I think the behaviour could differ compared to before. We should check this.