Publish block data to external services via webhooks

[AdarshRon]

What does this implement/fix? Explain your changes.

This PR adds a DataPublisher utility class to the nightfall client, that is used to publish L2 block data to external services via webhook. This is enabled via the environment variable CALL_WEBHOOK_ON_CONFIRMATION. The url of the webhook is configurable via the environment variable WEBHOOK_PATH.

The DataPublisher supports multiple destinations(webhook for now, could be different transport mechanisms in future like grpc or socket), including multiple webhook destinations for fallback/backup.

The PayloadSigner class abstracts away the optional signing of the payload using HMAC. The signing key(secret) is configurable via the environment variable WEBHOOK_SIGNING_KEY.

The RetryHandler class abstracts away the retry mechanism with exponential backoff strategy that is configurable via environment variables CALL_WEBHOOK_MAX_RETRIES and CALL_WEBHOOK_INITIAL_BACKOFF.

Does this close any currently open issues?

No

What commands can I run to test the change?

• Add the required environment variables to docker-compose.dev.yml:

   environment:
       - CALL_WEBHOOK_ON_CONFIRMATION=true
       - WEBHOOK_PATH=http://<webhook_url>
       - WEBHOOK_SIGNING_KEY=<secure_signing_key>

• Perform tokenisation/mint and wait for layer 2 block to be confirmed.
• Observe the published data on a destination webhook url.