Fix Multiple Access Token feature customization failure

[NethmiRanasinghe]

Purpose

This PR fixes an issue in Multiple Access Token feature customization. Enabling the renew_token_without_revoking_existing config causes the token issuance flow to skip the existing token check, bypassing thess custom logics.

Implementation Approach

Make the handleTokenBinding method in JWTTokenIssuer class protected so that anyone can extend this method to modify their customizations.

Fixes: wso2/api-manager#4063

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.51%. Comparing base (3d03e45) to head (14cb0a1).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2909      +/-   ##
============================================
- Coverage     57.52%   57.51%   -0.01%     
+ Complexity     9052     9006      -46     
============================================
  Files           669      669              
  Lines         50688    50695       +7     
  Branches      11111    11010     -101     
============================================
- Hits          29160    29159       -1     
- Misses        17474    17481       +7     
- Partials       4054     4055       +1     

Flag	Coverage Δ
unit	40.23% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.