add configurable CE metadata extension prefix

[cchayka-campsci]

Proposed Changes

• allow the CloudEvent extension attributes "partition" and "offset" to have a customizable prefix

As brought up before in this issue #3992, a previous version of NodeJS cloudevents would throw an error for any extension > 20 chars in length.

We are in a tough position to update this package for a large number of legacy services, so we were hoping you could consider allowing this change.

Basically just adding a new env var "CE_METADATA_EXTENSION_PREFIX" to change the "knativekafka" part of the "knativekafkapartition" and "knativekafkaoffset" CE extensions.

Release Note

:broom: allow customization of the prefix of the CE extensions

Docs

previous pull request to the wrong branch: #4144

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: cchayka-campsci
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[linux-foundation-easycla]

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[knative-prow]

Hi @cchayka-campsci. Thanks for your PR.

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[pierDipi]

/ok-to-test