Ensure live query subscription unsubscribes

[dplewis]

New Feature / Enhancement Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.

Feature / Enhancement Description

The SDK ensures that live query subscribes successfully with a promise. The same should be applied to unsubscribe. This could be used in the SDK test suite to ensure connections to the server are closed.

Example Use Case

Using a promise would be a breaking change but would be aligned with subscribe

await subscription.unsubscribe();

Using a callback is an alternative

subscription.unsubscribe((error) => {});

Edit: I think we can support both

[parse-github-assistant]

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