Add CometMock specific functionality: Advancing Time, Downtime & Advancing Blocks

[p-offtermatt]

Problem

CometMock should eventually provide specific functionality like advancing time, choosing signers and proposers,
running empty blocks, etc.
These custom endpoints should eventually be used in our e2e tests if we need more advanced testing behaviour.

This issue deals with three functionalities:

• Advancing Time
• Advancing Blocks
• Causing downtime

Closing criteria

The end-to-end tests make use of CometMocks custom endpoints to advance time, advance blocks, and invoke downtime.

Related issues

#938 the issue for implementing this functionality in CometMock