Design how to get certificates from `PerasCertDB` for diffusion

[amesgen]

The outbound side of certificate diffusion needs a source of certificates to serve. The PerasCertDB needs to expose appropriate functionality for this use case. The goal of this ticket is to design and implement this interface.

Already existing approaches for similar tasks:

• TxSubmissionMempoolReader is the mechanism used by the outbound TxSubmission logic.
• Follower is the mechanism used by the ChainSync server to send updates to the chain (MsgRollForward/MsgRollBackward).

We want a similar pull-based mechanism for the PerasCertDB to subscribe to new certificates being added.