Add CCU transaction creation and posting on sidechain from mainchain

[ishantiw]

Description

Chain connector plugin maintains all the data and create a CCU that is used to post updates from Sidechain to mainchain. But we need to post CCU from mainchain to sidechain as well to maintain the liveness of mainchain on the sidechain.

Acceptance Criteria

• Plugin should be able to post updates from mainchain to sidechain.
• Should have all the unit test to validate CCU similar to CCUs being posted on mainchain

Additional Information

• Most of the implementation for CCU creation is done from sidechain to mainchain and the same can be used for mainchain to sidechain updates.
• Further read LIP0053: Posting Cross-chain Updates on Mainchain and on Sidechains
• Create or reuse the _newBlockHandler of sidechain for the mainchain CCU creation and posting