Light consensus client/node

[kostko]

We should support a light consensus client so that compute nodes which are not validators don't need to sync and execute everything.

Requires #1898 to make it easy to do remote consensus queries after verifying the headers and getting the state roots.

Related to #2349 to avoid compute nodes from doing irrelevant work.

Linked Issues

• Basic consensus light client support
• Tendermint light client provider (part of go/consensus/tendermint: Add support for state sync #2882).
• Consensus light client interface should expose read-only state
• go/consensus/tendermint: Refactor internal event handling
• Make sure TendermintService is only used internally
• Introduce the notion of consensus backend mode
• Add non-blocking consensus SubmitTx variant
• Move SubmitEvidence to LightClientBackend
• Light version of the Tendermint consensus backend which uses the light client internally.