release-23.1: changefeedccl: repopulate request body for webhook sink retries

[blathers-crl]

Backport 1/1 commits from #119326 on behalf of @rharding6373.

/cc @cockroachdb/release

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The webhook sink reuses an http request for retries. However, the http library consumes the request body, so retries may result in an error due to a non-zero content length but zero length body. This change re-initializes the http request body on every request so that it will have the right contents on every retry.

Epic: none
Fixes: #118485

Release note (bug fix): Fixes a bug in the webhook sink where the http request body may not be initialized on retries, resulting in the error "http: ContentLength=... with Body length 0".

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Release justification: Fixes a bug in the webhook sink that can result in no request body after certain retryable errors.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
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• All backports must be reviewed by the owning areas TL and one additional
  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
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• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is