[feature request]: optional description for application variables

[radu-matei]

The variable schema in spin.toml should have an optional description field. This field would be used by tools that render the application manifest and help operators and developers understand the application post-deployment.

spin/crates/manifest/src/schema/common.rs

Lines 10 to 37 in 0a4c05a

	/// The name of the application variable.
	#[derive(Clone, Debug, Serialize, Deserialize, JsonSchema)]
	#[serde(deny_unknown_fields)]
	pub struct Variable {
	/// Whether a value must be supplied at runtime. If not specified, required defaults
	/// to `false`, and `default` must be provided.
	///
	/// Example: `required = true`
	///
	/// Learn more: https://spinframework.dev/variables#adding-variables-to-your-applications
	#[serde(default, skip_serializing_if = "is_false")]
	pub required: bool,
	/// The value of the variable if no value is supplied at runtime. If specified,
	/// the value must be a string. If not specified, `required`` must be `true`.
	///
	/// Example: `default = "default value"`
	///
	/// Learn more: https://spinframework.dev/variables#adding-variables-to-your-applications
	#[serde(default, skip_serializing_if = "Option::is_none")]
	pub default: Option<String>,
	/// If set, this variable should be treated as sensitive.
	///
	/// Example: `secret = true`
	///
	/// Learn more: https://spinframework.dev/variables#adding-variables-to-your-applications
	#[serde(default, skip_serializing_if = "is_false")]
	pub secret: bool,
	}

[lann]

As discussed in the Spin meeting today, we might consider using TOML ## Doc Comments for this sort of thing. This isn't a standard convention in TOML but I think it should be possible with some combination of toml::Spanned and/or toml_edit.