Implement BlockBackend and HeaderBackend for Client type.

[timwu20]

Issue Summary

Taken from design doc:

I propose implementing the BlockBackend and HeaderBackend into the newly introduced Client. Given that db.Backend already provides access to the underlying HeaderBackend this is trivially implemented by calling the Backend.Blockchain() which returns an impl of blockchain.Backend which embeds HeaderBackend. BlockBackend is implemented by also calling methods on the returned impl blockchain.Backend from Backend.Blockchain().

After the introduction and implementation of the BlockBackend and HeaderBackend interfaces, we can create a translation shim type to map the above BlockState methods to utilize the Client methods for both BlockBackend and ChainHeaderBackend.

Acceptance Criteria

• Introduction of BlockBackend interface
• Implementation of BlockBackend in Client.
• Implementation of HeaderBackend in Client.
• Tests

Other Information and links

• Substrate reference to BlockBackend trait.