Implement From<&'a KeyPair> for PrivateKeyDer<'a>

[LebedevRI]

I've stumbled into this when trying to write a simple rustls-driven server with self-signed cert.

The "obvious" and safe way to go from KeyPair to PrivateKeyDer seems to round-tripping via PEM:

rustls::pki_types::PrivateKeyDer::from_pem_slice(
            signing_key.serialize_pem().as_bytes(),
        )

But rcgen::key_pair::KeyPair::serialized_der() is

/// Returns a reference to the serialized key pair (including the private key)
/// in PKCS#8 format in DER

and PrivateKeyDer can be constructed from such a input.
This avoids memory allocation and PEM roundtripping,
both of which are optional features anyways,
and this conversion is non-failing.

Perhaps it makes sense to provide this QOL interface?