rpc: remove HTTP proxy support

[mzabaluev]

Closes #1342 by removing all use of hyper-proxy, and the HTTP proxy support in tendermint-rpc client which requires it.

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Added entry in .changelog/

[thanethomson]

@tony-iqlusion, IIRC you needed HTTP proxy support, which drove the work in #820 right?

[tony-iqlusion]

Yes, though it seems hyper-proxy may be unmaintained, unfortunately.

[romac]

How bad is that vulnerability in practice? I would expect that people will use proxies that they know and trust, and worst case that their VM running Hermes coming to a halt in case the proxy server is compromised to not have major consequences but I could be wrong. So perhaps before removing code that might be helpful to some of our users, although I am always partial to deleting code and tidying up dependencies, we should (a) assess the actual severity of the vulnerability for our use case and (b) see if we can wait it out a bit and push for the community to move to a better maintained alternative. What do you think @mzabaluev @tony-iqlusion @thanethomson?

[mzabaluev]

@romac I have commented on the issue. Indeed the potential for exploiting this is quite limited. I've also started looking into implementing HTTP CONNECT transport on our own, which does not seem like a big thing to do (even though we need to set aside a day or two for this).

[mzabaluev]

Let's preserve the functionality and go with #1362 instead.