Handle empty instance_config_version in topology elements

[claudia-correia]

Description

This changes the type of the instance_configuration_version field to *int32 for all topology elements and marks the fields as nullable.

Related Issues

Rel: https://elasticco.atlassian.net/browse/CP-5181

Motivation and Context

We can't set instance_configuration_version to 0 because it will be considered as an empty value and it will be omitted during JSON marshalling.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improves code quality but has no user-facing effect)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Readiness Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

[dimuon]

LGTM