test: extra webhook params

[Justintime50]

Description

• Adds WebhookCustomHeader model, allowing custom_headers to be passed when creating/updating a webhook
• Corrects payload wrapping for updating a webhook
• Bumps dependencies

Testing

• Tests new webhook params

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)

[keheitz]

LGTM!