feat: initial implementation of flashblocks sub

[steph-rs]

Proposed solution, addressing #118

Implementation:

• New base_subscribe RPC endpoint with newFlashblocks subscription kind
• Returns full RpcBlock on each flashblock update (similar to eth_subscribe("newHeads"))
• Follows reth's EthPubSub patterns using jsonrpsee subscriptions and broadcast channels
• Only available when flashblocks enabled via --websocket-url flag

[haardikk21]

I wonder if instead of a new RPC method, we should override eth_subscribe with special handling in case params includes the newPendingTransactions subscription type

Edit: nevermind, i guess that doesn't allow consumers to use logs as the subscription type then. we want logs based off of flashblock data

Edit 2: i just saw your comment on the issue

[steph-rs]

@danyalprout please let me know what you think🙏

[haardikk21]

Could you add some tests for this? Probably in tests/rpc.rs