[Feature] Add support for PKJWT grant flow and backend auth token management.

[isuruh15]

Description:

Currently Authorization header contains the end-user token and that need to be changed to have a separate access token which is obtained wrt to configured backend service.

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Affected Product Version:
WSO2 API Manager - 4.3.0 with WSO2 Healthcare Accelerator - 1.0.0

OS, DB, other environment details and versions:

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