Feature Request - allow configuring which host ld-proxy-relay binds to

[brycefisher]

Is your feature request related to a problem? Please describe.
When I run ld-relay-proxy on my mac as part of a test suite, I see a security popup because ld-relay-proxy binds to all network interfaces and is nonconfigurable.

Describe the solution you'd like
An environment variable that would allow me to force a specific bind address (in my case, only ever 127.0.0.1)

Describe alternatives you've considered
I could use the ldtest packages inside my application, but I'm trying to test the fully compiled binary, and I'd rather not change any application source code.

Additional context
I think I could probably contribute a small PR to this effect, depending on if any tests needed to be added, etc.

[cwaldren-ld]

Hi @brycefisher , this sounds like a reasonable request. If you wouldn't mind contributing a PR, that would be helpful.