use an enum for Target::kind and Target::crate_types

[oli-obk]

We know all possible variants, and can still include an Unknown(String) fallback to be more forward compatible.

The fields I'm talking about are

cargo_metadata/src/lib.rs

Lines 440 to 446 in 56fcbaa

	/// Kind of target ("bin", "example", "test", "bench", "lib", "custom-build")
	pub kind: Vec<String>,
	/// Almost the same as `kind`, except when an example is a library instead of an executable.
	/// In that case `crate_types` contains things like `rlib` and `dylib` while `kind` is `example`
	#[serde(default)]
	#[cfg_attr(feature = "builder", builder(default))]
	pub crate_types: Vec<String>,