add proxy support

[Yunuuuu]

Fix #221. The title summarizes the aim of the PR.

[gaborcsardi]

Thanks! But don't the libcurl proxy environment variables work already?

[Yunuuuu]

Since my network is heavily restricted due to my country's policies, I want to make usethis git-related functions work, which depend on gh and gert.

Having support for proxy environment variables in gh would be extremely useful, as usethis does not expose many arguments from gh or gert, making it difficult to configure network access directly.

I would really appreciate any advice on how to test whether the proxy environment variable is working correctly in gh. I couldn’t find any verbose argument for this—does one exist? Thanks in advance for your help!

[gaborcsardi]

See e.g. https://everything.curl.dev/usingcurl/proxies/env.html

[Yunuuuu]

Thank you for the reference. I have checked both gh and gert in relation to the usethis::use_github_release() function. The function related to gh works well, but the one related to gert does not. It seems that gert does not respect the Git config proxy or the proxy environment variables. For me, it's safe to close this pull request. However, if you think it’s necessary, It's happy to merge it to provide additional arguments for controlling the proxy in gh directly. Thanks again for your help!