Mock Drand to enable offline devnet

[geoff-vball]

Checklist

• [X] This is not a new feature or an enhancement to the Filecoin protocol. If it is, please open an FIP issue.
• [X] This is not brainstorming ideas. If you have an idea you'd like to discuss, please open a new discussion on the lotus forum and select the category as Ideas.
• [X] I have a specific, actionable, and well motivated feature request to propose.

Lotus component

• [X] lotus daemon - chain sync
• [ ] lotus miner - mining and block production
• [ ] lotus miner/worker - sealing
• [ ] lotus miner - proving(WindowPoSt)
• [ ] lotus miner/market - storage deal
• [ ] lotus miner/market - retrieval deal
• [ ] lotus miner/market - data transfer
• [ ] lotus client
• [ ] lotus JSON-RPC API
• [ ] lotus message management (mpool)
• [ ] Other

What is the motivation behind this feature request? Is your feature request related to a problem? Please describe.

Right now, devnets use the mainnet drand beacon. This means we cannot run a devnet without being connected to the internet.

Describe the solution you'd like

Mock the beacon to enable the use of devnets offline.

Describe alternatives you've considered

No response

Additional context

No response