Migrate base_subscribe to eth_subscribe

[danyalprout]

We added base_subscribe in #144. It would be great if we could move this from the Base namespace to the Eth namespace.

I wonder if we could have eth_subscribe(ExtendedSubscriptionKind), where ExtendedSubscriptionKind = {...SubscriptionKind, NewFlashblocks, PendingLogs }.

This would:

• Keep it consistent with the rest of the Flashblock API's (that override the underlying eth_ RPC's)
• Should be backwards compatible with the existing eth_subscribe setup
• We could potentially override the existing newPendingTransactions event to use the Flashblocks pending state