fix bug in revokeAccessToken method

[3nethz]

Purpose

Fixes an issue where the revokeAccessToken method does not include client_secret in the revocation request body, causing token revocation to fail for confidential clients with the error invalid_client. This change ensures the SDK supports both public and confidential clients by conditionally appending client_secret from the config if available.

Related Issues

• fixes revokeAccessToken method does not support confidential client authentication using client_secret #262

Related PRs

• N/A

Checklist

• Manual test round performed and verified.
• UX/UI review done on the final implementation.
• Documentation provided. (Add links if there are any)
• Unit tests provided. (Add links if there are any)

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines?
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets?