Cloud events json schema

[1123]

Description

This pull request adds an example for writing data respecting the Cloud Events Standard to Confluent Cloud, and registering the corresponding schema with Confluent Schema Registry. It is a somewhat minimal example for this requirement and independent of the other client samples in this repository.

Author Validation

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[x] Documentation: This sample comes with its own README.md file. Not sure if it needs to be documented elsewhere.
[x] clients/cloud: This is the directory where the sample has been added. It could be moved to another directory if needed. It should work just as well with an on-premise cluster.

This pull request does not touch any of the other sample programs, and thus should not affect their functionality.

Reviewer Tasks

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[ybyzek]

@1123 thanks for the PR.

1. The examples in the clients/cloud/ folder are for languages, not specifications. A better location may be to put this in clients/ in a new subfolder called cloudevents/?
2. Should there be a consumer example?
3. If there isn't already, perhaps there should be a link added from docs.confluent.io
4. Please recommend another reviewer who is familiar with CloudEvents, who can review this PR from that perspective.

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