feat: state.SnapshotTree interface for drop-in replacement

[ARR4N]

Why this should be merged

Allows for a drop-in replacement of snapshot.Tree (i.e. one that uses block hashes instead of state roots). This is intended as a temporary solution while we investigate having the state root affected by the block hash to remove path ambiguity.

How this works

Introduction of:

1. state.SnapshotTree interface to match methods required on snapshot.Tree as used by state.StateDB; and
2. stateconf package for variadic options plumbed by StateDB.Commit() through to SnapshotTree.Update().

Although variadic (to maintain function call-signature compatibility) only the stateconf.WithUpdatePayload(any) is expected to be used. Recipients of the options can access the payload with stateconf.ExtractUpdatePayload().

How this was tested

Unit test demonstrating propagation of stateconf.UpdateOption payload.