Make near-workspaces::Contract::new method public

[ilblackdragon]

When testing factory contracts, there is no easy way in near-workspaces to turn a account id into a Contract because both ::new and ::account methods are only crate level.

[frol]

@ilblackdragon Thank you for opening the issue! Neither new nor account methods will help you as Contract type requires the signer. So account_id won't be enough. We are moving towards deprecating near-workspaces and using a unified near-api-rs instead. Please, consider trying out that crate. Just for your reference, here is our migration to near-api-rs & near-sandbox-rs for sputnik-dao tests: near-daos/sputnik-dao-contract#231