Add proxy for http client

[Milky2018]

Add a proxy for http client.

• The proxy is implemented in proxy.mbt
• Update README.mbt.md
• Add a commented test at proxy_test.mbt, it can be verified when developers have a proxy server

[coveralls]

Pull Request Test Coverage Report for Build 796

Details

• 7 of 33 (21.21%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.1%) to 80.164%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/http/request.mbt	5	7	71.43%
src/http/client.mbt	2	5	40.0%
src/http/proxy.mbt	0	21	0.0%

Totals
Change from base Build 794:	-1.1%
Covered Lines:	1370
Relevant Lines:	1709

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💛 - Coveralls

[Guest0x0]

testing can be done by:

• set up a local TCP server
• use @http.ServerConnection::new to wrap a TCP connection and handle the CONNECT request
• discard the @http.ServerConnection and inspect the underlying TCP connection for tunneled traffic

[Guest0x0]

I am not sure if this abstraction is necessary. For @http.Client::connect and @http.get, the protocol is always known. So protocol based proxy selection should be done manually by the user IMO, and @http should just accept an optional (host, port) pair for proxy

[Guest0x0]

Also, I think we should leave some room for future HTTPS proxy support (i.e. the CONNECT request itself is TLS encrypted) in the API design

[Guest0x0]

My idea on API design:

• add a method @http.Client::connect, which sends a CONNECT request, and turn the @http.Client into a TCP tunnel on success
  • this API could be private at first, but is useful on its own (tunnel non-http traffic through HTTP proxy)
• make the proxy argument another plain @http.Client. Internally, .connect() will be called on that client

This way we get:

• HTTPS proxy support for free
• proxy authentication etc. for free: the user can do whatever @http.Client support on proxy creation, such as passing custom headers

[Guest0x0]

close via #199