feature: HttpClient health-check in default cache implementation

[newtork]

Context

Related issue https://github.com/SAP/cloud-sdk-java/issues/970

Adding a health check that uses reflection to check boolean flag on connection pool whether it's shutdown or not.

Feature scope:

• [ ] Confirmed to be working by user
• [ ] No unexpected behavior change from E2E tests, etc.

Definition of Done

• [x] Functionality scope stated & covered
• [x] Tests cover the scope above
• [x] ~Error handling created / updated & covered by the tests above~
• [x] ~Documentation updated~
• [ ] Release notes updated