[Keylime] Support push model for agent attestation

[nate-double-u]

Keylime enables users to monitor remote nodes (file integrity and measured boot) using a hardware based cryptographic root of trust. Keylime currently operates on a pull basis which means that the tenant or verifier connect to the agent to collect attestation data. This works fine in most virtualized environments where all the devices are in the same network, but not for edge devices or in BYOD contexts. This work would allow remote nodes to work in a "push" model instead of the normal "pull" model.

Mentor(s): Thore Sommer (@THS-on), Michael Peters (@mpeters), Marcio Silva (@maugustosilva)

https://github.com/cncf/mentoring/blob/main/summerofcode/2022.md#support-push-model-for-agent-attestation