outbound-networking (mostly) refactoring

[lann]

A hodgepodge of refactoring extracted from a larger (but now smaller) change:

• Tweak/add doc comments
• Fix spin_expressions Template Display
• Replace HostConfig::List(Vec<String>) with HostConfig::Literal(url::Host)
  • List was only ever exactly one item
  • url::Host provides validation and normalization
• Add HostConfig::literal(host)
• Reorganize error handling a bit
  • This does change some validation error messages, making them more consistent but also introduces some use of anyhow Context which is more verbose
• Add AllowedHostConfig::is_for_service_chaining, replacing logic in spin-loader
• Capricious rename HostConfig::new test helper to ::unwrap_literal