Using KeyMap and DescriptorSecretKey for signing transactions

From an outsider point of view KeyMap looks like interesting way to handle keys after importing descriptor.

Let's say for example we have such descriptor where we supply tprv which will be imported into KeyMap

"wsh(t:or_c(pk(02a7d30ac6b0cd55b6868f5a65aff1dbdaa18f4315fe04de809fffb2899340ae0f),v:multi(1,[255f0d9b]tprv8ZgxMBicQKsPerMTN7nhwCpqaAEPkaBCDeE8c9ekAY1xhPmF3HM9r3oz33V8JTwsJYp9FYLDQumLzNssLo6UNMNesVwPP1KcgA5atrSqU2w/84'/1'/1/0/0,02f17cce1778c101e3b0ac5c76997af3752b0a221b1ebffc23af679328f3f5d6fc,02350b5a9a12df96579d150f3f751de416e58a67f2ba9bc99f6964b7a48a79042b)))"

We could use KeyMap later in similar loop

        for (public, private) in &keys {
                  // ... doing something with keys
        }

The problem though is that public and private are DescriptorPublicKey and DescriptorSecretKey objects do not really contain convenient API even for extracting keys and using them in signing. I have a code example and honestly I am not entirely sure I didn't something wrong. Besides It doesn't actually work and I think the problem is that there is just wrong key in &prv.xkey.private_key.

        for (pk, prv) in &self.keys {
            match prv {
                DescriptorSecretKey::Single(prv) => {
                    let sig = self.secp.sign_ecdsa(&msg, &prv.key.inner);
                    let public = &prv.key.inner.public_key(&self.secp);
                    assert!(self.secp.verify_ecdsa(&msg, &sig, public).is_ok());
                    psbt.inputs[0].partial_sigs.insert(
                        public.to_public_key(),
                        bitcoin::EcdsaSig {
                            sig,
                            hash_ty: hash_ty,
                        },
                    );
                },
                DescriptorSecretKey::XPrv(prv) => {
                    let sig = self.secp.sign_ecdsa(&msg, &prv.xkey.private_key);
                    let public = &prv.xkey.private_key.public_key(&self.secp);
                    println!("{}", &public);
                    assert!(self.secp.verify_ecdsa(&msg, &sig, public).is_ok());
                    psbt.inputs[0].partial_sigs.insert(
                        public.to_public_key(),
                        bitcoin::EcdsaSig {
                            sig,
                            hash_ty: hash_ty,
                        },
                    );
                },
            };
        }

Start with the KeyMap and DescriptorSecretKey APIs referenced in the issue, then inspect the supplied Rust signing example and the XPrv handling. Determine how imported descriptor keys are intended to map to signing keys and what API behavior is missing; the issue names no files, tests, or concrete completion criteria.