Add Polkadot Keygen

[dandanlen]

Description

Our CFE is already capable of generating polkadot keys but we don't currently emit a request to do so.

Worth waiting for or branching off #2218 for this.

Something like:

• Add required ChainApi members for vault rotations (SetAggKeyWithAggKey equivalent).
• impl VaultRotator for (T1, T2) where T1: VaultRotator, T2: VaultRotator (or equivalent). This should ensure that we wait until the keygen for both chains is successful before emitting the rotation transaction.
• Add a new polkadot pallet instances for vault, threshold signer and broadcaster, link everything together.

Out of scope:

The CFE doesn't have to be able to send the rotation transaction or witness anything yet.

The main thing is being able to generate a new polkadot AggKey. The can 'confirm' it without actually broadcasting anything.

Relevant people

@kylezs @ramizhasan111 might be worth looking at this together.

[msgmaxim]

Created #2288 as there is still a bit of work left before CFE is ready to process Polkadot requests. The work can mostly proceed in parallel though we probably shouldn't actually start sending polkadot requests before #2288 is merged. At the same time #2288 requires polkadot vault/events to be added so we can "subscribe" to them.