Improve DX for `IdentityInput`

[arboleya]

Discussed in #1844

^{Originally posted by Dhaiwat10 March 7, 2024}
Take this Sway function signature for example:

fn new_game(player: Identity);

If you try to call this function via the TS SDK, it would look something like this:

await contract.functions.new_game({ Address: { value: walletAddress });
await contract.functions.new_game({ ContractId: { value: contractId });

The idea is to change it to something like:

type Identity = {address: string, contractId?: never} | {address?: never; contractId: string}

await contract.functions.new_game({ address: walletAddress });
await contract.functions.new_game({ contractId: contractId });

[nedsalk]

It seems to me that we can generalize this approach for all enums, not just the Identity one?
For example, this is currently possible:

// call-test-contract
    const { value } = await contract.functions
      .take_b256_enum({ Value: 'qweqwe', Data: 'asds' })
      .call();

But we'd want to only provide Value or Data.