feat(ethereum): introduce get_block_header to fetch L1 block headers

[t00ts]

(Self-explanatory title)

This is part of #3160.

My goal overall is to have an always-running background task that continuously fetches and updates gas price data. As I move forward in integration, I might (will surely try to) merge this with our current L1 sync task to avoid bloating our runtime with yet another task, but for now, I really wanted to just get this done as an independent method.

I've run some manual tests on this method and it appears to work as expected.

[t00ts]

Update: I also made get_finalized_block_number public as I think I'll need it later.

[kkovaacs]

Was wondering how we're going to use these...

If the goal is a background task that's fetching information from L1 then re-creating the Websocket connection seems quite a big overhead.

I was wondering if we could structure this similarly how we're fetching starknet core contract logs from Ethereum now:

• have a background task polling the latest finalized block
• have a subscription for blocks (provider.subscribe_blocks()) that stores not-yet-finalized block headers in memory

[t00ts]

I am well aware of this design issue and will address it in an upcoming PR.

Regardless of how we use this, the EthereumClient will keep a persistent connection that will be used for all methods.

@kkovaacs

[vbar]

Was wondering how we're going to use these...

If the goal is a background task that's fetching information from L1 then re-creating the Websocket connection seems quite a big overhead.

IMHO we should get a better idea what information will be needed, first. Looking at https://github.com/starkware-libs/sequencer/tree/main/crates/apollo_l1_gas_price , it only needs the prices every 15 minutes (yes, the interval is configurable, but surely there's some expected range), and more than that is hard to say, because it gets the going rate from some separate oracle (which I couldn't find in the repo but again, we can ask) - if Starkware plans to run that oracle publicly, we could just reuse it...

[t00ts]

Design issue was addressed in #3164

Will revisit the gas price related stuff in a future PR. Closing for now.