Use CloudNative.CloudEvents nuget package instead of creating our own models

### Description

Currently we use our own class implementing the CloudEvent type. This forces us to keep it in sync with the standard implementation with regards to what fields are mandatory and not. Currently we have wrong implementation of what fields are nullable causing issues when using this model.

### Additional Information

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### Tasks

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### Acceptance Criterias

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Start by locating the existing class that implements the CloudEvent type and all of its usages in the library. Compare those usages with the CloudNative.CloudEvents NuGet package, then verify that the replacement handles nullable and mandatory fields correctly and preserves existing behavior.

Written by the indexing model from the issue text.