[Signature] Add top level API.

Expose the image structure and policy structure in the crate root `lib.rs`.
The caller can use the signature library as follows:

```rust
use signature::{Image, Policy};
use oci_distribution::Reference;

let reference = Reference::try_from("docker.io/library/example:tag").unwrap();
let image_digest = "sha256:sha256:69704ef328d05a9f806b6b8502915e6a0a4faa4d72018dc42343f511490daf8a";

let policy = Policy::from_file("path/to/policy/file").unwrap();

let mut image = Image::default_with_reference(reference);
image.set_manifest_digest(image_digest)?;

policy.is_image_allowed(image).except("Validate image failed");
```

Signed-off-by: Jiale Zhang <[email protected]>

signature/src/lib.rs

@@ -9,3 +9,6 @@ extern crate strum;
 mod image;
 mod policy;
 mod signatures;
+
+pub use image::Image;
+pub use policy::Policy;