Export the `ToSign` structure from` morley-multisig`

Description

Problem: Multisig contracts create a structure that embedd the operation and some other data like the contract address, and verifies the incoming signatures on this data. If there is some miscommunication about this structure, then the good signatures can fail on the contract.

Solution: Export this structure from the multisig contract implementations so that the consumers will have a type safe way to form this ToSign structure for signining.

Related issue(s)

Resolves #

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