Download toolchains using proxy if one is set in the environment

[vsarunas]

Read http_proxy/https_proxy/HTTP_PROXY/HTTPS_PROXY standard environment variables; and set HTTPClient() proxy configuration with one of the values.

Fixes #108

@cmcgee1024 - this adds the minimal silent use based on the environment. Most tools read one of those variables.

[cmcgee1024]

@vsarunas thank you for this contribution. Overall it looks good.

Now that this functionality is in the core http client implementation, can you look at removing the ProxyHTTPRequestExecutorImpl implementation in the tests since it's doing a subset of the same things? The macOS CI will be able to verify the implementation since it uses a proxy internally.

Also, can you run the formatter on the code like this?

swift run swiftformat Sources Tests Tools Package.swift

[vsarunas]

@cmcgee1024 - done!

Needed to to change HTTPRequestExecutorImpl to be a class in the same was as in the tests to be able to shutdown the httpClient

[cmcgee1024]

@swift-ci test macOS

[cmcgee1024]

@vsarunas unfortunately, it looks like the macOS CI is down this week and also merging of PR's is blocked until next Monday. So, this will need to wait.

In the mean time, how do you feel about writing up a short howto guide about using these environment variables for http/https proxies? We've built up a few howto guides already, published on the spi site here: https://swiftpackageindex.com/swiftlang/swiftly/main/documentation/swiftlydocs

The source code for these guides can be found in Documentation/SwiftlyDocs.docc.

[cmcgee1024]

@swift-ci test macOS

[vsarunas]

@cmcgee1024 , sure! Added a small paragraph in the getting started page.

[cmcgee1024]

This looks good. Thank you @vsarunas for your contribution!