New proxy type for NominationPools

[kianenigma]

A new variant that allows NominationPools calls.

Not quite urgent, since most UIs don't support proxy that well yet.

[kianenigma]

cc @rossbulat

[kianenigma]

@Ank4n this is probably a 5 min job, just being forgotten :D

[xlc]

Should we just have a proxy variant Pallet(PalletIndex) and Call(PalletIndex, CallIndex) and automatically be 80% future proof with new pallets?

[kianenigma]

I have always been a fan of more flexible proxies that allow for specific call indices instead of matching on specific pallets in the runtime, but there has been push-back that for Polkadot we want to be conservative.

That being said, now with paritytech/substrate#12891, I think we have more reasons to allow this.

[Ank4n]

Should we just have a proxy variant Pallet(PalletIndex) and Call(PalletIndex, CallIndex) and automatically be 80% future proof with new pallets?

Created a separate issue for this here
paritytech/polkadot-sdk#700