Make creating BlockchainDbMeta easier

[mattsse]

This type is only used to identify the Cache:

https://github.com/foundry-rs/alloy-fork-db/blob/e286594d83134a1191a97bbfb3a102a589f06706/src/cache.rs#L432-L432

this isn't very good, but we can make creating a SharedBackend easier at least by having helper functions on BlockchainDbMeta

alloy-rs/examples#145

[yash-atreya]

Closed by #22