Replicated Security - support key assignment feature

[jmc000]

For Replicated Security there are 2 ways to sign blocks. The first and simplest one is to use the same priv_validator_key.json for provider and consumer chains. The problem with this solution is that it increases the insecurity of the Hub, and more in the future as RS scale.

As describe in this blog post:

validators should use the key assignment feature so that they can keep their provider chain and consumer chain keys separated.

Key assignment feature allows validators to use different keys for consumer and provider chains by broadcasting to the network which consensus key they will be using to validate a consumer chain (providerd tx provider assign-consensus-key <consumer_pub_key>)

For the launch of first consumer chain (Neutron) its still manageable to use the same key, but I think in the future it could be great that Horcrux supports RS key assignment!