Add Multiple providers

[LeoPatOZ]

Resolves #6 #142

[0xNeshi]

I'm wondering whether it makes sense to construct the RobustProvider separately, and then pass that to the event scanner builder 🤔 The provider has a lot of configs, and it's possible the configs may increase in number.

Current API mixes provider and scanner configs:

let scanner = EventScannerBuilder::historic()
    .from_block(500)
    .fallback_ws(ws_url)
    .fallback_ws(fallback_url)
    .max_retries(5)
    .timeout(100)
    .build()?; 

Alternative separates them:

let provider = RobustProvider::<Ethereum>::new()
    .ws(ws_url) // or: `.add_websocket`, `.add_ipc`, that might be even better
    .ws(fallback_url)
    .max_retries(5)
    .timeout(100)
    .build()
    .await?;

let scanner = EventScannerBuilder::historic()
    .from_block(500)
    .provider(provider)
    .build()?; // maybe even this becomes sync

This would even allow the scanner to support both alloy's Provider and our own RobustProvider (depending on what the scanner user needs):

// maybe the dev needs something simple

let provider = RootProvider::<Ethereum>::new(...);

let scanner = EventScannerBuilder::historic()
    .from_block(500)
    .provider(provider)
    .build()?; 

[LeoPatOZ]

Actually was debating this but ended up choosing this way - which i see now is actually not great.

I think we should go with passing in the RobustProvider directly instead of muddling this api with both the scanner and provider methods.