This allows callers to verify certificates using algorithms that Go
does not support (yet).

For instance, here we're verifying the ML-KEM-512 example certificate
from the LAMPS WG signed by a ML-DSA-44 public key.

    https://github.com/lamps-wg/dilithium-certificates/blob/main/examples/ML-DSA-44.crt
    https://github.com/lamps-wg/kyber-certificates/blob/main/example/ML-KEM-512.crt

package main

import (
        "crypto/x509"
        "crypto/x509/pkix"
        "encoding/asn1"
        "encoding/pem"
        "errors"
        "fmt"
        "github.com/cloudflare/circl/sign/schemes"
        "os"
)

func loadCert(path string) (*x509.Certificate, error) {
        raw, err := os.ReadFile(path)
        if err != nil {
                return nil, fmt.Errorf("ReadFile(%s): %w", path, err)
        }
        block, _ := pem.Decode(raw)
        if block == nil {
                return nil, fmt.Errorf("pem.Decode(%s) failed", path)
        }
        return x509.ParseCertificate(block.Bytes)
}

func main() {
        dsaCert, err := loadCert("ML-DSA-44.crt")
        if err != nil {
                panic(err)
        }
        kemCert, err := loadCert("ML-KEM-512.crt")
        if err != nil {
                panic(err)
        }

        roots := x509.NewCertPool()
        roots.AddCert(dsaCert)
        _, err = kemCert.Verify(x509.VerifyOptions{
                Roots: roots,
                UnknownAlgorithmVerifier: func(alg pkix.AlgorithmIdentifier,
                        signed, signature, pk []byte) error {
                        if !alg.Algorithm.Equal(asn1.ObjectIdentifier{2, 16, 840, 1, 101, 3, 4, 3, 17}) {
                                return errors.New("unsupported scheme")
                        }
                        scheme := schemes.ByName("ML-DSA-44")
                        ppk, err := scheme.UnmarshalBinaryPublicKey(pk)
                        if err != nil {
                                return err
                        }
                        if !scheme.Verify(ppk, signed, signature, nil) {
                                return errors.New("invalid signature")
                        }
                        return nil
                },
        })
        if err != nil {
                panic(err)
        }
}