Add the proxy pallet to the runtime

[vstam1]

Description

This PR adds parity's proxy pallet to the runtime. The Proxy Pallet introduces a pivotal functionality within Subtensor, empowering users to establish a proxy account for another, enhancing both security and usability in key management. This versatile feature allows users to configure proxies with distinct privileges through the ProxyType. Presently, two essential proxy types have been incorporated:

Any: This proxy type grants the proxy account the authority to execute any extrinsic on behalf of the associated account, offering comprehensive control.

Staking: Specifically designed for managing staking capabilities, this proxy type restricts the proxy account to executing add_stake and remove_stake extrinsics for the associated account. It proves invaluable for scenarios where a hotkey is employed to oversee staking activities for a cold wallet.

New ProxyTypes can be added in the future to add more fine-grained control over the proxy account's privileges.

[vstam1]

@Rubberbandits Would love to see this feature added to the runtime, is this something you guys are open to? Let me know if I should change/add something. It would allow us to let institutional parties manage our assets, while we are still in control of staking/unstaking operations.

[scucchiero]

This would be very important for me as well.

[camfairchild]

Thanks for the contribution! Closing in favour of #286