Add light sync mode to l2geth

[qbzzt]

Is your feature request related to a problem? Please describe.

It is takes hours to do a full sync with Kovan, and that number is likely to increase.

Describe the solution you'd like

Allow l2geth to do a faster sync, either snap or light (https://pkg.go.dev/github.com/ethereum/[email protected]/eth/downloader#SyncMode)

[tynes]

We currently do not support snap or light syncing but will eventually. I'd imagine for the 1.0.0 release

[smartcontracts]

Very likely that this will be supported in 1.0.0. Keeping this issue open for posterity.

[smartcontracts]

Transferring to specs repo. I assume this will be included as part of Bedrock, but the specs repo is the right place to have visibility on this request, otherwise this may be forgotten.

[protolambda]

We plan to support snap, but it will only be for the happy-case sync. If there are no L2 nodes to sync from (network issues, DoS, no connection or otherwise), you can still sync by deriving block-by-block from L1 data.

Also note that to trigger snap sync in geth, we need to present a full block header of the point to sync towards to geth, not just a block-hash. When we get unsafe (i.e. via L2 p2p) block distribution we can add either gossip of undisputed headers, or some simple request-response type of protocol to retrieve this from other nodes.