feat: add socks5 proxy support

[kartikaysaxena]

Fixes #471

This adds support for a socks5 proxy via a new flag

[tstirrat15]

@kartikaysaxena the changes look good to me, but I'm not that familiar with this part of the gRPC client. Were you able to test this locally and see that it works?

[kartikaysaxena]

@kartikaysaxena the changes look good to me, but I'm not that familiar with this part of the gRPC client. Were you able to test this locally and see that it works?

yeah tried with local socks5 proxy server using github.com/armon/go-socks5 and then ./zed schema write root.zed --proxy=socks5://127.0.0.1:8000 --insecure and it forwards that request to spicedb endpoint

[tstirrat15]

LGTM. Thank you!