Debug server port forwarding

Summarizing @itowlson's description of the problem:

https://github.com/bytecodealliance/StarlingMonkey/pull/218 started adding support for running a JS debugger in the guest, which requires that the debugger be able to make an outbound socket connection to a debug server (typically on localhost). Spin's allow_outbound_hosts security mechanism makes this somewhat awkward in practice, because a developer would need to either always allow connections to some local port (whether actively debugging or not) or would have to e.g. edit the spin.toml while debugging, which is annoying and error prone.

My proposal (originally here):

• Pick a domain namespace like *.spin-debug.alt (.alt is a TLD reserved for this sort of thing)
• Configure debugger(s) to point at fixed address(es), e.g. js.spin-debug.alt:9229
• Teach spin up a new flag like --debug-server js=127.0.0.1:9229
• Spin resolves js.spin-debug.alt to some appropriate reserved IP internally and forwards guest connections to that IP to the configured matching --debug-server (note js from the --debug-server value above)

Start with the spin up command and its allow_outbound_hosts handling; the issue does not name specific files or tests. Review how guest outbound connections are resolved, then verify that a --debug-server mapping supports the proposed debug namespace and forwards connections to the configured local address.