Add gRPC + grpc-gateway routes as alternative to legacy tendermint REST endpoints

[clevinson]

The SDK currently exposes the following REST endpoints as thin wrappers over tendermint:

• GET /node_info
• GET /syncing
• GET /blocks/latest
• GET /blocks/{height}
• GET /validatorsets/latest
• GET /validatorsets/{height}

In keeping with how we are marking all other non GRPC-gateway based REST endpoints as deprecated (#7686), we need to decide if these endpoints should also be considered deprecated, and if so, what alternative endpoints we communicate users to use instead for Stargate chains.

The options as I see them:

1. Leave endpoints as is (undeprecated), and continue supporting these endpoints for Stargate, not deprecating until we have a bigger aligned plan with tendermint's future use of grpc and/or grpc gateway
2. Mark as deprecated and write GRPC + GRPC services exposing the same functionality via standard Cosmos SDK grpc(+gateway) interfaces
3. Mark as deprecated and tell clients to query tendermint RPC directly for these queries

Would be good to hear from someone on the Tendermint side about this as well-- possibly @marbar3778 ?

cc @amaurymartiny (see original comment here)

[amaury1093]

(2) seems like the way to go to me 👍

I don't think it's super high priority to add these gRPC services now, but it would be a good task for 0.40.1.

[alexanderbez]

My vote is for (2), and sooner rather than later 👍