[MD] Add fetch_and_decrypt()

[noCharger]

This function will fetch the credential by id and return the decrypted field

[noCharger]

Exposing the fetch and decrypt() method is less recommended because we need to import credential management service to core, plus we need to take saved object client as a dependency. Given that core.dataService already requires a saved object client as a dependency for fetching data sources, it's logical to get credential object on data service layer and only use decrypt() as a utility method.

Talked with @zhongnansu offline and we came to an agreement.