feat: add new `ContractOutputType` `ExpectationSignature`

_Related: https://github.com/OpenAEV-Platform/openaev/issues/5446_

Please replace every line in curly brackets { like this } with appropriate answers, and remove this line.

## Problem to Solve
Add new output type: `ExpectationSignature` for use in ContractOutputElement

## Current Workaround
None

## Proposed Solution
Add constant for use in ContractOutputElement
