feat(Usage Reports): add billing snapshot config validate service to usage reports

[manukm-ibm]

PR summary

Adding new API to validate s2s authentication in billing snapshot configuration service in usage reports. TRI: https://github.ibm.com/BSS/architecture/blob/master/design-docs/services/billing/apptio-integration.md Issue: https://github.ibm.com/BSS/metering-deliverables/issues/2102

PR Checklist

Please make sure that your PR fulfills the following requirements:

• [x] The commit message follows the Angular Commit Message Guidelines.
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

Current vs new behavior

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[manukm-ibm]

Integration test results of new changes

Examples results of new changes

[ibm-devx-sdk]

:tada: This PR is included in version 0.55.0 :tada:

The release is available on GitHub release

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