Function certify 

pub async fn certify(
    authority: &KeyPair,
    certificants: &[String],
    policies: Option<&Value>,
) -> Result<Value, SeaError>

Build and sign a capability certificate.

Arguments

• authority — The keypair of the authority granting rights
• certificants — List of pubkeys being authorized (the “c” field)
• policies — Optional JsonValue with recognized keys: e, r, w, rb, wb

Returns a signed certificate in Gun.js format: {m: ..., s: ...}