Client: proxy support

[arthuraraujo-msft]

Support proxies when making the connection to SFS.

Automatic handling should be done during the web requests with the goal of finding the first proxy that enables the connection to be created.

The handling will depend on the HTTP library chosen, but a possible approach is through libproxy/libproxy (github.com), which is also used by the Lite DO Client in the DO SDK.

• This library does the handling automatically depending on the current platform and returns a list of candidate addresses that should be attempted in order. Here's how DO uses it.
• The need to use a library arises from the fact that each platform does proxy configuration in a different manner. Windows uses the IE configuration, or the netsh proxy configuration. Some Linux applications read from the environment variable http_proxy. MacOS has a system-wide proxy configuration similar to Windows.

libproxy has a good example with libcurl: https://github.com/libproxy/libproxy/blob/main/docs/samples/libcurl/curlget.c

[arthuraraujo-msft]

Check if cert pinning works through the proxy as well
CONNECT would be used
https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/CONNECT