[WIP] Add support for API extension reference expansions

[Copilot]

• Explore codebase and understand API extension resource implementation
• Investigate SDK versions and reference expansion feature
• Update SDK to v1.9.0
• Update provider meta structure to expose raw HTTP client for raw API calls
• Add expand field to extension trigger schema
• Implement custom types for extensions with expand support
• Update create/update/read operations for extension resource
• Add tests for the new expand field
• Update documentation
• Run tests and fix any issues

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add support for API extension reference expansions</issue_title>
<issue_description>https://docs.commercetools.com/api/releases/2026-03-12-added-reference-expansion-in-api-extensions

Commercetools just added support for reference expansions in API extensions.
Would be great if that can be added to the API extension resources in the provider</issue_description>

Comments on the Issue (you are @copilot in this section)

@mvantellingen @copilot pick this up?

• Fixes Add support for API extension reference expansions #635

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