[ic-agent]async sign function in Identity trait

[hide-yoshi]

async sign function in Identity trait

Identity#sign is a synchronous function, but it should be async.
This is because the function may need to perform I/O operations, such as using AWS KMS or another service to sign the message keeping the private key secure.
Do you think it would be better to make the sign function async?
For example, I'd like to add another implementation of Identity trait for AWS KMS like this:

trait Identity {
    fn sender(&self) -> Result<Principal, String>;
    fn public_key(&self) -> Option<Vec<u8>>;
    async fn sign(&self, message: &[u8]) -> Result<Vec<u8>, Error>;
}


#[derive(Clone)]
pub struct KmsIdentity {
    client: Client,
    key_id: String,
    public_key: Vec<u8>,
}

impl KmsIdentity {
    pub async fn new(client: Client, key_id: String) -> Self {
        let public_key = client
            .get_public_key()
            .key_id(key_id.clone())
            .send()
            .await
            .unwrap()
            .public_key
            .unwrap()
            .as_ref()
            .to_vec();
        KmsIdentity {
            client,
            key_id,
            public_key,
        }
    }
}

impl Identity for KmsIdentity {
    fn sender(&self) -> Result<Principal, String> {
        ...
    }

    fn public_key(&self) -> Option<Vec<u8>> {
        Some(self.public_key.clone())
    }

    async fn sign(
        &self,
        content: &ic_agent::agent::EnvelopeContent,
    ) -> Result<ic_agent::Signature, String> {
        let result = self
            .client
            .sign()
            .key_id(self.key_id.clone())
            .signing_algorithm(SigningAlgorithmSpec::EcdsaSha256)
            .message(Blob::new(content.to_request_id().signable()))
            .send().await.map_err(|e| e.to_string())?;
        let public_key = self.public_key().unwrap();
        let sig = Signature {
            delegations: None,
            public_key: Some(public_key),
            signature: Some(result.signature().unwrap().as_ref().to_vec()),
        };
    }
}

[hide-yoshi]

main...hide-yoshi:agent-rs:main
This is the patch