Add getter for auto-stake hotkey and make auto-staking per-netuid

[mcjkula]

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

The auto-staking implementation as it is has two limitations (IMO):

• Miners cannot check what their auto-stake destination is set to. There is a set_coldkey_auto_stake_hotkey but no corresponding getter, which would also be ideal for the BTCLI/SDK.
• The auto-stake is currently implemented globally across all subnets, it means miners can only set one auto-stake destination for their entire coldkey. This forces miners to choose a single validator across all subnets they mine on (assuming they use the same coldkey), but a miner may want to auto-stake on netuid 1 but not on netuid 50. It also causes their auto-stake to get no dividends should the set destination not CHK or not be a validator on the subnet, while it may be on a different subnet (where the miner also mines).

Describe the solution you'd like

• Implement get_coldkey_auto_stake_hotkey(coldkey, netuid) that allows miners to query their current auto-stake destination for a specific subnet.
• Modify the auto-stake destination to work per-netuid instead of globally

Describe alternatives you've considered

No response

Additional context

I also think that the auto-stake destination should be the miner/UID's hotkey by default, then they can change it on the netuid, and to turn it "off" they would set it back to their miner's hotkey