Implement MainchainRegistrationCommand for sidechain interoperability module

[ishantiw]

Description

For SidechainInteroperabilityModule,

• Implement verify(context: CommandVerifyContext): VerificationResult of MainchainRegistrationCMD as per explained in LIP0043:Mainchain registration#verification
• Implement execute(context: CommandExecuteContext): void of MainchainRegistrationCMD as per explained in LIP0043:Mainchain registration#execution
• All the current block header related checks and verification steps should be moved to execute() as we don't have access to current block in the verify hook

Acceptance Criteria

• Unit tests for all the code paths for verify()
• Unit tests for all the code paths for execute()
• Make sure entry in all the subStores is created for a sidechain