Support for SubIssues events and AlertRule schema fix

[JamieMagee]

Resolves #751

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Before the change?

• AlertRule.Tags was incorrectly a string instead of IEnumerable<string>
• No support for SubIssues events

After the change?

• Correct types for AlertRule.Tags
• Support for SubIssues events

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

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