Implement custom JWT assertion signing (#1001)

[arvindkrishnakumar-okta]

Issue(s)

https://github.com/okta/okta-sdk-java/issues/878

Description

Thanks @clementdenis for the original PR https://github.com/okta/okta-sdk-java/pull/1001

Category

• [ ] Bugfix
• [x] Enhancement
• [x] New Feature
• [ ] Library Upgrade
• [ ] Configuration Change
• [ ] Versioning Change
• [ ] Unit or Integration Test(s)
• [ ] Documentation

Signoff

• [ ] I have submitted a CLA for this PR
• [ ] Each commit message explains what the commit does
• [ ] I have updated documentation to explain what my PR does
• [ ] My code is covered by tests if required
• [ ] I did not edit any automatically generated files