feat(ext/fetch): Parse unix socket and vsock proxies from env var

[lrowe]

Allows configuring a default unix/vsock proxy for upstream requests.

[bartlomieju]

@lrowe can you give some context on this change?

[lrowe]

Configuring a default upstream unix/vsock proxy is generally useful for cases where you are deploying deno behind a forward proxy and you want to enable requests to sibling endpoints behind the same forward proxy and/or control or cache upstream requests. Having this option for the default HTTP client makes for a cleaner separation between developer and ops concerns.

This is particularly interesting for me because I want to run deno behind kvmserver for per-request isolation in which case making a new connection to a forward proxy listening on a unix socket has much lower overhead than to a reverse proxy on a tcp socket which incurs additional overhead in terms of the network and the extra roundtrip for the CONNECT request.