fix(types): mark type parameters as optional

[wolfy1339]

Some types which end-users could be using, declare the type parameters as requried, when in fact they are entirely optional.

This arises when users try to use the event handler code on it's own

Resolves #ISSUE_NUMBER

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Before the change?

• Types which had optional type parameters had them set as required

After the change?

• The types now have the type parameter set to optional and default to unknown
  This change shouldn't introduce any noticeable behaviour changes for users who use the default Webhooks class

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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