Add a relayers endpoint to get the estimate when NEAR -> Ethereum transaction will be relayed

[alexauroradev]

This will improve indication on the frontend for how long the user should wait for sync

[mfornet]

There are two places two update:

1. Near Client on Ethereum. This contract should expose:
   • currentHeight: Heights that considered finals.
   • nextHeight: Next height that will become final. (Can be equal to currentHeight if there is no new block relayed)
   • timeToNextHeight: Timestamp when nextHeight is considered final.

There is a little difference between (currentHeight/nextHeight) vs (trustedHeader.height / untrustedHeader.height). Since untrustedHeader.height can become final before relaying a new block.

This information can be used by tools (such as the UI) to determine the expected time such that action on Ethereum is considered final.

2. Near2Eth relayer

In the relayer we should expose the frequency at which blocks are being relayed, which is the next block height, and at when it will be submitted.

[alexauroradev]

It seems that NEAR Client already is showing the correct info. See here: https://github.com/near/rainbow-bridge/blob/master/contracts/eth/nearbridge/contracts/NearBridge.sol#L195-L213

So only the Near2Eth should be updated.