Introduce Externalities interface. Utilize OverlayedChanges in new Ext type that fulfills Externalities

[timwu20]

Issue Summary

From design doc:

Second, we create a new type that implements TrieState which combines the read-only TrieBackend of the block hash, as well as the OverlayedChanges type that can be extracted before calling AddBlock. This new TrieState will be passed into the runtime instance. This introduced implementation of TrieState is analagous to the substrate primitive sp_statemachine::ext::Ext and the analgous trait to the TrieState interface is sp_externalities::Externalities.

Acceptance Criteria

• Introduce Externalities interface.
• Implement Externalities in a new type analogous to Ext.
• Unit tests

Other information and links

• Substrate reference to Externalities trait.
• Substrate reference to Ext type.