Reward pallet RPC

[theodorebugnet]

A few questions I'm blocked on at this point:

1. Build failure. This looks very similar to what I was getting when failing to implement the RPC in the runtimes, but here I've definitely added it to all four parachain runtimes and the standalone. Could you take a look please?
2. See comment below regarding possibly cleaning up the cast into Balance
3. We have two instances of the reward pallet, once using VaultId as the RewardId for vault block rewards, one using AccountId for escrow rewards. Right now I'm just trying to get the VaultId one to work for starters, as it's also going to be the main one useful for simplifying the lib, but I think it would be good to have both available as RPCs - how would I define that? The naive way of just adding a second impl to the runtimes doesn't work, and I'm not familiar enough with the inner workings of substrate to figure out how to approach this.

[gregdhill]

1. Missing trait bound on RuntimeApiCollection - see this commit.
2. Missing cast on pallet to Rewards trait - see this commit.
3. Split compute_reward on RewardApi into compute_escrow_reward and compute_vault_reward - see this commit.

[sander2]

Minor nits. Did you test this? When adding a new pallet to the rpcs, it's easy to miss required additions in all of the boilerplate