Add docs for HTTPS_PROXY and NO_PROXY

[samuelallan72]

Many cloud environments require a proxy to access external resources. Proxy support would be required in parca-agent in order to run it in these environments.

It doesn't appear possible to configure parca-agent to use a http/https proxy currently?

[samuelallan72]

Note: as discussed, it does actually support the HTTPS_PROXY and NO_PROXY ( https://github.com/grpc/grpc-go/blob/master/Documentation/proxy.md ). :)

It would be great to add this to the parca-agent docs, and also add support for configuring proxy for the snap service.