feat: Add eth_syncing RPC method

[fridrik01]

Fixes: #10622

This PR adds eth_syncing as a new RPC method to Lotus. This method returns an object with data about the sync status or false.

NOTE: I added SyncApi as dependency to the EthModule in order to implement this functionality, not sure if there was any way around that...

NOTE: If calling eth_syncing as soon as lotus daemon starts (within a <5 seconds) then it will result in an error (see test plan) as the syncing hasn't started properly, but I am to making it instead wait.

Test plan

Started my lotus daemon on mainnet from snapshot and calling this method produces the following output depending on the sync status:

Before starting sync:

curl http://localhost:1234/rpc/v1 -X POST -H "Content-Type: application/json" --data '{"method":"eth_syncing","params":[],"id":1,"jsonrpc":"2.0"}'
{"jsonrpc":"2.0","id":1,"error":{"code":1,"message":"no active syncs, try again"}}

During sync:

curl http://localhost:1234/rpc/v1 -X POST -H "Content-Type: application/json" --data '{"method":"eth_syncing","params":[],"id":1,"jsonrpc":"2.0"}'
{"jsonrpc":"2.0","result":{"startingblock":"0x2a9e8e","currentblock":"0x2a9e95","highestblock":"0x2a9ea8"},"id":1}

After sync is done (according to spec "result" should be set to "false" in this case):

url http://localhost:1234/rpc/v1 -X POST -H "Content-Type: application/json" --data '{"method":"eth_syncing","params":[],"id":1,"jsonrpc":"2.0"}'
{"jsonrpc":"2.0","result":"false","id":1}

[maciejwitowski]

Can you explain this? Why are looking for the index of the last non-idle one from ActiveSyncs and eg not the 1st one? (I'm not sure what ActiveSyncs represent tbh)

[fridrik01]

I wanted to have the same logic as when calling lotus sync wait which is done like this (see https://github.com/filecoin-project/lotus/blob/master/cli/sync.go#L286)

There is also this commit which picks the last worker in case all are idle, but it does not go in details as to why

[Stebalien]

LGTM with one bug.