Add fingerprint / thumbprint support

Hi! I try to use the `authenticode-rs` crate for a security application. I'm interested in parsing the signer certificates of PE files and validating their thumbprint/fingerprint.

I'm able to get the certificates from the authenticate signature, but there seems to be no option to get the fingerprint of the certificates, either for SHA-1 or SHA-256.

The authentic ode-parser crate exports the functionality here: https://docs.rs/authenticode-parser/0.5.0/authenticode_parser/struct.Certificate.html#method.sha1 and here https://docs.rs/authenticode-parser/0.5.0/authenticode_parser/struct.Certificate.html#method.sha256

Is there any chance to add the functionality to the crate or export the certificate in a way that the hash can be computed?