Allow validators that are not participating in consensus on the provider to validate consumer chains

[p-offtermatt]

Problem

Currently, only validators in the active set on the provider can validate on consumer chains, which limits the number of validators that can participate in Interchain Security (ICS).
Validators outside of the active set might be willing
to validate on consumer chains, but we might not want to make the provider validator set larger, e.g. to not put more strain on the consensus engine.
This runs the risk of leaving consumer chains with too few validators.

Closing criteria

We have implemented a way to let validators that are not part of consensus on the provider chain
validate on consumer chains.

Problem details

See this draft ADR for more context: https://github.com/cosmos/interchain-security/pull/1879/files