all: support proxy of twilio base urls

[expertonium]

Adds support for proxying calls through a specific outbound server.

[kevinburke]

Thanks for this. I'm a bit nervous about this since it might make extracting tokens easier - if you can figure out how to set an env var in someone's environment you could get them to send traffic to a totally different host without their expecting that. Maybe that's a dumb thing to worry about.

[kevinburke]

How about a new method SetURLsFromEnv() that you can call to override BaseURL, MonitorURL etc based on the env vars you have picked out?

[expertonium]

Yeah, I hear you 100% about concerns with cross-contamination within a container where someone isn't aware of the implications, or perhaps even the fact of, Something Else setting/overriding global ENV vars in such a way as proxies Twilio data Somewhere Else.

Setters make sense to me.

I confess, it's an inconvenience, because with env vars I can just set them on a container, and then fill-in-the-blank application using your package just works. In this case Zitadel, where I'm presently doing a replace with the PR at issue here:
zitadel/zitadel@9840e27

Zitadel and others like it might be best served by an actual Admin UI Field that allows override of the Twilio base url, and thereby uses the Setters you're describing, in a highly intentional, well-communicated way.

Anyway, it all hinges on your notion of Container Security and who is responsible for it, with the context that your module is included in other open-source projects in a big way.