Introduced LTV-SDO evidence

We have added several new features for Sign API v2, including support for LTV-SDO evidence, end-user notifications, and a customer-facing events channel.

This release includes the following updates:

• Single user, signing a single document producing an LTV-SDO (long-term validation signed data object) as evidence.
• End-user notifications when an action is needed.
• Customer-facing events channel for signing session state changes and creation.