Flesh out variables implementation

[rylev]

Uses spin-expressions to more fully implement the variables interface.

This also provides a new interface in tests for setting application variables. For example:

With this in the spin.toml

[variables]
secret = { required = true }
# ...
[component.my-component.variables]
my_secret = "{{secret}}"

And the following in the test:

spin_test_sdk::bindings::fermyon::spin_test_virt::variables::set("secret", "PASSWORD");

The following in the Spin app:

let value = variables::get("my_secret").unwrap_or("unknown".to_string());
println!("VALUE: {:?}", value);

The following would be printed:

VALUE: "PASSWORD"

[fibonacci1729]

Looks good!