Implement headers support in getCredentials and awaitCredentials

[poovamraj]

Changes

This change adds support for headers in getCredentials and awaitCredentials of CredentialsManager and SecureCredentialsManager

References

https://github.com/auth0/Auth0.Android/issues/698

Testing

• [ ] This change adds unit test coverage
• [ ] This change adds integration test coverage
• [ ] This change has been tested on the latest version of the platform/language or why not