Ehtereum chain attestor

[gjermundgaraba]

The API might change somewhat, but currently there is a relatively simple API in sidecar/attestators/attestator/chainattestator.go that is used to trigger collection of claims and the endpoint that the grpc server uses to get the latest claim available.

The goal of the chain attestation is to query packet commitments on-chain, sign them and deliver them when queried.

Since the Ethereum implementation is most likely only going to be useful for IBC Eureka, the attestor should simply find a height it trusts (that is finalized) and query the IBC Eureka contract (which is still being implemented, but can be mocked for now).