[design] prepare design for acp-77/subnet-staking

[ceyonur]

Prepare initial design docs for subnet-staking with acp-77 (contracts, interfaces, flows)

[ceyonur]

My general plan is to follow current version of ACP-77 and put up a sample implementation. This is also important for ACP-77 itself as we can provide feedbacks/request design changes according to the contract.

A high-level roadmap is:

• Start with PoA contract with capability of adding validators (no staking)
• Write up an interface/abstract contract with functions and signatures
• Decide on warp message payloads/formats
• Put up an initial implementation
• Test out the contract with warp + mock relayer
• Suggest feedbacks for ACP-77
• Test out with in a PoC branch/master with ACP-77 activated