feat(ethereum): reuse provider on query methods

[t00ts]

Every query method (get_chain, get_starknet_state, etc.) currently creates a new WebSocket connection, uses it once, and discards it. This was OK when we were using http, but for ws it seems quite wasteful.

On my first attempt to solve this, I tried establishing the connection in the EthereumClient::new() method, which meant having an async constructor.

This worked, but it cascaded heavily through the entire codebase. A bunch of test helpers had to bcome async, context builders, etc.

I have this in a separate branch if anyone wants to check it out. But I just felt it was too much of a change for something that was supposed to be an internal optimization.

So, yanked it all, started again, this time going for lazy initialization of the connection, keeping new() sync and failing later if the connection couldn't be established.

We now have

1. sync_and_listen which for simplicity and isolation I decided to leave alone.
2. A shared connection/provider stored in Arc<RwLock<Option<WsProvider>>> for all the query methods.