Feature Request: Need a way to configure forward proxy settings for injected sidecars

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## Feature Request
There should be a way to configure forward proxy settings for the injected sidecars.

### What problem are you trying to solve?
We're trying to get Linkerd working in one of our production clusters, which is behind our corporate firewall. Anything that wants to get to the public internet must use our forward proxy. The linkerd-proxy sidecar doesn't use this, so applications which have it injected into their pod are unable to reach the internet.

### How should the problem be solved?
We need a way to configure the linkerd-proxy sidecar to use our forward proxy server, either via environment variables (HTTPS_PROXY etc) or some other mechanism. This configuration should allow for NO_PROXY settings as well.

### Any alternatives you've considered?
We could change the firewall rules on our production cluster to allow all internet traffic out. This is very undesirable.

### How would users interact with this feature?
This could be passed to the linkerd CLI tool by parameters. Something like:
`linkerd install --forward-proxy-https=my-proxy.corp.net --forward-proxy-noproxy=127.0.0.1,.corp.net,.svc | kubectl apply -f -`

Start with the linkerd CLI install entry point and the injected linkerd-proxy configuration, then trace how install parameters become sidecar settings. Done means forward-proxy and NO_PROXY values can be configured for injected sidecars and the resulting manifests work with kubectl.

Written by the indexing model from the issue text.