snapshot + db backend modifications for use by coreth & subnet-evm

[darioush]

Why this should be merged

• subnet-evm / coreth use block hash based snapshots,
• subnet-evm / coreth use modified hashdb (and later pathdb) backends so we can pass these in

How this works

This pull request makes changes to the core/state and triedb packages to enhance the flexibility of the state management and database handling. The changes involve introducing new interfaces, updating function implementations.

Changes to core/state:

• Introduced the snapshotTree interface to abstract the snapshot handling in StateDB (core/state/statedb.go).
• Updated the StateDB struct to use the new snapshotTree interface (core/state/statedb.go).
• Modified the New function to handle snapshotTree and ensure proper handling of nil pointers (core/state/statedb.go). <-- not sure if there is a better way to handle this

Changes to triedb:

• Refactored the Config struct in triedb/database.go to use interfaces for HashDB and PathDB (triedb/database.go). -- Trying to keep changes minimal esp. to tests.
• Introduced new interfaces Backend, HashBackend, and PathBackend to define methods for different state schemes (triedb/database/database.go).
• Updated the NewDatabase function to use the new Backend interfaces and handle nil configurations (triedb/database.go).
• Simplified various methods in triedb/database.go to use the new Backend interfaces (triedb/database.go) [1] [2] [3] [4] [5] [6] [7] [8] [9].
• Added New methods to Config structs in triedb/hashdb/database.go and triedb/pathdb/database.go to instantiate HashBackend and PathBackend respectively (triedb/hashdb/database.go) [1] (triedb/pathdb/database.go) [2].

How this was tested

CI