feat(triedb): expose backend for custom use (#183)

## Why this should be merged

To override the standard trie behavior for custom databases in
`core/state/database.go`, custom methods may need to be defined on the
backend database.

## How this works

Exposes `backend` interface for the API

## How this was tested

No tests necessary - isn't used in `libevm`.

Author: alarso16

triedb/database.libevm.go

@@ -51,6 +51,11 @@ type DBOverride interface {
 	ReaderProvider
 }
 
+// Backend returns the underlying backend of the trie database.
+func (db *Database) Backend() BackendDB {
+	return db.backend
+}
+
 func (db *Database) overrideBackend(diskdb ethdb.Database, config *Config) bool {
 	if config.DBOverride == nil {
 		return false