Configuration for Spin applications

[radu-matei]

Configuration could be split into:

• language / toolchain specific configuration — (Cargo.toml, package.json)
• Spin configuration

We currently don't have any configuration defined for applications. This could include (the list below does not represent a single configuration file, but potentially information aggregated from multiple sources):

• static assets to include in the component
• runtime configuration (volumes, allowed HTTP domains)
• routes
• overrides for dependencies
• Hippo configuration

We need to learn from the current configuration splits in Hippo, Wagi, and yo-wasm and surface a coherent user experience for handling configuration.

[radu-matei]

A first version for this was merged in #26.
Closing, in favor of more specialized issues for configuration sources.