fn der_encode_ecdsa_p256_sig(raw: &[u8]) -> Result<Vec<u8>>
Convert raw R || S (64 bytes for P-256) to DER Ecdsa-Sig-Value SEQUENCE { r INTEGER, s INTEGER } - what rcgen expects from a RemoteKeyPair::sign for ECDSA P-256.
R || S
Ecdsa-Sig-Value SEQUENCE { r INTEGER, s INTEGER }
RemoteKeyPair::sign