Add set_children and swap_coldkey/hotkey to proxy settings.

[vstam1]

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

The current proxy settings are not up to date, and new features are needed to enable safe proxy use for validators. I would like to see either the set_children extrinsic added to the Staking proxy or a new proxy type. Currently, the most secure option for a set_children proxy would be NonFungibile, however, this would still allow the swap_coldkey/hotkey functions, making my coldkey vulnerable to attacks by using this proxy type.

Describe the solution you'd like

• Add new proxy type for set_children or add set_children to Staking proxy.
• Add swap_coldkey/hotkey to NonFungibile as these functions could be used indirectly to get control over a cold/hot key

Describe alternatives you've considered

No response

Additional context

No response

[camfairchild]

In mainnet from #1016