feat: Add java sample app to authenticate client certificate

[nxhafa]

Description

Provides a sample java code to help users to start with client certificate authentication. This sample app uses client-cert keystore to construct SSL context and then execute a request against auth/login endpoint on the gateway service.

Linked to # (issue)
Part of the # (epic)

Type of change

Please delete options that are not relevant.

• fix: Bug fix (non-breaking change which fixes an issue)
• feat: New feature (non-breaking change which adds functionality)
• docs: Change in a documentation
• refactor: Refactor the code
• chore: Chore, repository cleanup, updates the dependencies.
• BREAKING CHANGE or !: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the style guidelines of this project
• PR title conforms to commit message guideline ## Commit Message Structure Guideline
• I have commented my code, particularly in hard-to-understand areas. In JS I did provide JSDoc
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• The java tests in the area I was working on leverage @nested annotations
• Any dependent changes have been merged and published in downstream modules

For more details about how should the code look like read the Contributing guideline

[sonarqubecloud]

Quality Gate failed

Failed conditions
1 Security Hotspot
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud