Add option to send oauth client credentials in body

[jochenott]

1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

#2508

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am modifying authorization mechanisms
   • I am adding new credentials
   • I am modifying OCSP code
   • I am adding a new dependency

3. Please describe how your code solves the related issue.

It adds an option to send the oauth client credentials in the request body, as defined in RFC-6749 Section 2.3.1 (as alternative).

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

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I have read the CLA Document and I hereby sign the CLA

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1 out of 2 committers have signed the CLA.
✅ (sfc-gh-pczajka)[https://github.com/sfc-gh-pczajka]
❌ @jochen-ott-by
_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}

[sfc-gh-fpawlowski]

Left a few nits

[jochenott]

I have read the CLA Document and I hereby sign the CLA

[sfc-gh-pczajka]

Thanks for the contribution!