Child Coldkey Delegation instead of Child Hotkey Delegation

[CapriciousSage]

Is your feature request related to a problem? Please describe.

With validators needing to split each subnet to a different hotkey under their coldkey for each subnet, child hotkey delegating is going to turn into a logistical nightmare, where it will be difficult to know which actual validator you're Child Delegating to. To solve this, we propose changing Child Hotkey Delegations to be Child Coldkey Delegations - associating the "CHK" with the coldkey instead of the hotkey, bypassing the "which hotkey of that validator to I chk to on SNXX".

This is Part Three of a Three Phase hotkey overhaul to strengthen validator security and resolve validator rate limits (where each hotkey can only set weights on one subnet in any given block).

Part One - Hotkey Splitting: https://github.com/opentensor/subtensor/pull/1559

Part Two - Prevent hotkey registration if already registered in any subnet: https://github.com/opentensor/subtensor/issues/1619

Describe the solution you'd like

By designating the Coldkey instead of the hotkey, it becomes easier to identify which validator you are Child Delegating to, and easier to scale/replicate as the target is the same on all subnets. This helps increase transparency and reduce administrative work.

Describe alternatives you've considered

While removing the ratelimit/limitation where each hotkey can only set weights on one subnet per block would allow validators to continue using a single hotkey for longer, there would be unintended consequences of this. It also does not resolve the security issue whereby a single compromised hotkey would compromise every subnet the validator operates on.

Additional context

This must be completed either with or after Part Two ("prevent hotkey registration if already registered in any subnet"), as it could become quite messy working out where the stake 'goes' if the validator has multiple hotkeys registered on the same subnet.