[OB3] Prevent overriding MTLS Cert Header at TokenFilter

[imesh94]

[OB3] Prevent overriding MTLS Cert Header at TokenFilter

When a proxy is added to IS APIs through gateway, TLS is terminated at gateway and the Gateway transport certificate is passed to the IS. This causes client authentication failures. The certificate is added to the request as a header by GatewayClientAuthenticationHandler. This PR adds changes to the TokenFilter to prevent overriding that header if it is available.

Issue link: https://github.com/wso2-enterprise/ob-compliance-toolkit-cds/issues/511

Doc Issue: Optional, link issue from documentation repository

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Development Checklist

1. Built complete solution with pull request in place.
2. Ran checkstyle plugin with pull request in place.
3. Ran Findbugs plugin with pull request in place.
4. Formatted code according to WSO2 code style.
5. Migration scripts written (if applicable).

Secure Development Checklist

1. Ran FindSecurityBugs plugin and verified report.
2. Ran Dependency-check plugin and verified report for new dependencies added.
3. Ran Dependency-check plugin and verified report for dependency version changes.
4. Have you verify the PR does't commit any keys, passwords, tokens, usernames, or other secrets?
5. Have you followed secure coding standards in WSO2 Secure Engineering Guidelines?

Testing Checklist

1. Written unit tests.
2. Documented test scenarios(link available in guides).
3. Written automation tests (link available in guides).
4. Verified tests in multiple database environments (if applicable).
5. Verified tests in multiple deployed specifications (if applicable).
6. Tested with OBBI enabled (if applicable).
7. Tested with specification regulatory conformance suites (if applicable).

Automation Test Details

Test Suite	Test Script IDs
Integration Suite	TCXXXXX, TCXXXX

Conformance Tests Details

Test Suite Name	Test Suite Version	Scenarios	Result
Security Suite	VX.X	Foo, Bar	Passed

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