Verkle-db backends

[kevaundray]

This is a tracking issue for the following tasks:

• impl SqLite as a key value and make the database optional
• One can also impl Sqlite in verkle repo and use tables
• Possibly add https://github.com/nomic-io/merk as a backend

[ControlCplusControlV]

Backends should also likely be moved to rust feature flags, so that downstream consumers can avoid compiling costly impls (such as rocksDB) if they only plan on using one

[kevaundray]

The verkle-db crate has the backends under a feature flag here: https://github.com/crate-crypto/rust-verkle/blob/master/verkle-db/Cargo.toml#L10

I should clarify that consumers do not need to implement or even use verkle-db since verkle-trie just needs somebody to implement the Db traits. I added verkle-db to make it easier for those that did not want to. In that light, we could make verkle-db optional, though not super important