[SLS-19] Rest client implementation for Identity Service

[svariant]

List of Changes

• Generated rest client using openAPI generator
• Defined methods that retrieves the idp certification data based on the given entityId and assertion issue instant
• Written tests to validate IdpCertSimpleClient class methods
• Removed unused classes from assertion-rest-client-native & identity-service-rest-client-native
• Defined two configuration classes for the respective ApiClient of IdpCertSimpleClient & AssertionSimpleClient

Motivation and Context

Issue SLS-19 Rest client implementation for Identity Service

How Has This Been Tested?

We've written tests for the following events:

• Retrieving successfully a SPID certification
• Retrieving successfully a CIE certification
• Verify that the correct error is shown when given a non-existing entityID
• Verify that the correct error is shown when given a wrong instant for SPID certifications
• Verify that the correct error is shown when given a wrong instant for CIE certifications

Types of changes

[X] Chore (nothing changes by a user perspective)
[X] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)