Function load_signer 

pub fn load_signer(
    path: &Path,
    key_id: &str,
) -> Result<FileEd25519Signer, String>

Read the PKCS#8 Ed25519 key at path as the key named key_id.

The key stays in this process. Only the signature it makes goes on the wire.

Errors

Returns a message when the file cannot be read, and when it does not hold a PKCS#8 Ed25519 key.