Add sign_message function to AccountKeys

[DOBEN]

Purpose

I am in the process of adding new test cases to the sponsored transaction smart contract. These new test cases generate AccountKeys and Signatures within the test cases. To avoid having to convert signature map types, this PR adds a function sign_message that returns an AccountSignatures type. This simplifies the test cases as follows:

let account_signatures: AccountSignatures = keypairs
        .sign_message(&to_bytes(&message_hash));

Changes

• Add sign_message function to AccountKeys