Function create_grant_token 

pub fn create_grant_token(
    local_secret: SecretKey,
    remote_id: EndpointId,
    max_age: Duration,
    capability: Caps,
) -> Result<Rcan<Caps>>

Create an rcan token that grants capabilities to a remote endpoint. The local endpoint is the issuer (granter), and the remote endpoint is the audience (grantee).