Add disk_offering & override_disk_offering to instance resource

[vishesh92]

Fixes #68
Changes in these fields is ignored as of now.

[Copilot]

Pull Request Overview

This PR adds support for disk offering configuration in CloudStack instance resources by introducing two new optional parameters: disk_offering and override_disk_offering. These fields allow users to specify disk offerings for virtual machines when deploying from templates or ISO images.

Key changes:

• Added two new optional fields to the instance resource schema with ForceNew behavior
• Implemented parameter handling in the instance creation logic
• Added documentation for the new fields explaining their usage with templates vs ISO images

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
website/docs/r/instance.html.markdown	Added documentation for the new disk_offering and override_disk_offering parameters
cloudstack/resource_cloudstack_instance.go	Added schema definitions and creation logic for the new disk offering fields

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