Non-blocking API

[chanced]

A non-blocking API would be great. The crate is not that ergonomic to use in a project that relies on the tokio runtime. This is due to the fact that you can not spawn a tokio::Runtime from within a runtime.

    #[tokio::test]
    async fn test_construction() -> Result<(), Box<dyn std::error::Error>> {
        tink_aead::init();
        dotenv::dotenv().unwrap();
        let key_uri = std::env::var("GOOGLE_KMS_KEY_URI")?;
        let credential_path_var = std::env::var("GOOGLE_APPLICATION_CREDENTIALS").unwrap();
        let credential_path = std::path::Path::new(&credential_path_var);
        let client =
            tink_gcpkms::GcpClient::new_with_credentials(&key_uri, credential_path).unwrap();
        let backend = client.get_aead(&key_uri).unwrap();
        // ...
        Ok(())
    }

thread 'key_manager::tests::test_construction' panicked at 'Cannot start a runtime from within a runtime. This happens because a function (like `block_on`) attempted to block the current thread while the thread is being used to drive asynchronous tasks.'

Also, dyn <Primitive> are cumbersome without Send + Sync.

/// Returns a [`tink_core::Aead`] primitive from the given keyset handle.
pub fn new(h: &tink_core::keyset::Handle) -> Result<Box<dyn tink_core::Aead>, TinkError> {
    new_with_key_manager(h, None)
}

I can get around the first issue with an actor and spawning a thread, but lack of Send + Sync on the primitives is a tough one.

I believe there are a few changes that would need to be made:

1. Create a feature flag, something like "non-blocking" to avoid breaking changes.
2. Add async-trait as a dependency
3. Add an AeadEnvelope trait:

#[async_trait::async_trait]
pub trait AeadEnvelope {
    async fn encrypt(
        &self,
        plaintext: &[u8],
        additional_data: &[u8],
    ) -> Result<Vec<u8>, TinkError>;

    async fn decrypt(
        &self,
        ciphertext: &[u8],
        additional_data: &[u8],
    ) -> Result<Vec<u8>, TinkError>;
}

4. A good bit of the dyn <Primitive> will likely need to be dyn 'static + <Primitive> + Send + Sync
5. Possibly provide implementations for the integrations but that's not as important as the above.

Thank you for your work and effort porting this.