fix: validate/coerce multisig safeTxGas,baseGas and nonce values to numbers

[iamacook]

Summary

A recent change to the Transaction Service, changed safeTxGas,baseGas and nonce values of multisig transactions to strings. This therefore broke validation.

This adjusts validation of the above fields, allowing both string/number and coerces the value to a string to not affect types client-side.

Changes

• Abstract previous schema change to related change on the Transaction Service to CoercedNumberSchema
• Use schema for safeTxGas, baseGas and nonce of multisig transactions
• Add appropriate test coverage

[falvaradorodriguez]

LGTM!

[iamacook]

As per discussion, this will be posthumously reviewed by @PooyaRaki as this is currently causing problems on staging.