Add SOCKS5 Proxy support for TCP

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master

• Issues fixed (if any): Bandwidth performance tool for chisel tunnel based on socks5 #1095 (partly)

• Brief description of code changes (suitable for use as a commit message):

SOCKS5 Proxy support for TCP. --socks5 option is added. Based on @mister-goo code suggested here.

The implementation is standalone, so there should be no change to functionality when the --socks5 is not used.

UDP is not supported since it requires added a header to each UDP packet sent. Since this header has to be taken into consideration in the iperf3 packet size (-l) it make the implementation more complex.

[bmah888]

Thanks for the pull request! SOCKS5 support is not something that we've seen as a requirement in the R&E networking space, so it's unfortunately a bit out of scope for iperf3's core functionality. We're also not able to test or maintain this functionality in our environment. This might be a good candidate for someone to integrate to a fork of iperf3.