Support socks proxies via local sockets instead of tcp sockets

[riastradh-brave]

Controlling permission for local sockets is much easier than controlling permission for tcp sockets. For Tor mode, we should support connecting to the Tor socks proxy via a local socket so that it is not accessible to anyone else on the system who can make tcp connections to 127.0.0.1:9150. This requires:

• teaching net/proxy about notation for local socket addresses, and
• teaching net/socket about making the socks connections via local sockets.

Related:

• Decide which Tor process to use browser-laptop#12935
• Protect connections between Brave and our Tor subprocess browser-laptop#12936