Trusted events

From the [JEP](https://github.com/jupyter/enhancement-proposals/pull/41), a conversation came up around being able to identify if events came from trusted components. For example, if a system is interested in capturing an audit trail of server activity, an important required would be to validate that these audit events came from a trusted source.

I'm writing down my thoughts on how the telemetry framework could make this easier

1. Provide a set of helper libraries to for publishers to sign events, and for consumers to validate the digest
2. Add fields in the `record_event` interface to allow event publishers to sign these events.
3. Provide other types of abstractions like a `SigningEventConsumer` or `SigningEventPublisher`to allow easier signing of events

cc @westurner @betatim @Zsailer @yuvipanda

Start by reading the linked JEP and then inspect the `record_event` interface and existing telemetry publisher and consumer entry points. The issue is done when the signing and validation abstractions, event fields, and trust model are agreed and implemented with coverage for trusted-event verification.

Written by the indexing model from the issue text.