[Filebeat][CEL] - Added support for Okta Oauth2 provider

[ShourieG]

Type of change

• Enhancement
• Docs

Proposed commit message

Added support for Okta as a separate OAuth2 provider. This is required because specific use cases
require access tokens with okta.* scopes, which involves a specific 2 step auth mechanism using a private
RSA JWK to generate a JWT which is in turn used to fetch an access token. Standard client_id & client_secrets don't
work in this scenario. It is a similar implementation to the httpjson PR: #36273. This implementation for CEL ensures feature parity with the httpjson input.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes Add Okta oauth2 provider support for CEL #36336

Use cases

Screenshots

Logs

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @ShourieG? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2023-09-06T10:55:10.743+0000

• Duration: 82 min 11 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3171
Skipped	176
Total	3347

💚 Flaky test report

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