Function derive_secret 

pub fn derive_secret(
    provider: &impl CryptoProvider,
    suite: CipherSuite,
    secret: &Hash,
    label: &[u8],
    transcript_hash: &Hash,
) -> Result<Hash, Error>

Derive-Secret as defined in RFC 8446 §7.1.

Derive-Secret(Secret, Label, Messages) = HKDF-Expand-Label(Secret, Label, Transcript-Hash(Messages), Hash.length)