feat(integration): add workspace_id support for workspace-scoped integrations

[roh26it]

Summary

Add support for creating workspace-level integrations by passing an optional workspace_id parameter to the portkey_integration resource.

• Add workspace_id (optional) parameter to create workspace-scoped integrations
• Add type (computed) attribute showing organisation or workspace
• Update client to include WorkspaceID in create request and response
• Add acceptance test for workspace-scoped integration
• Update documentation with examples for both integration types

Background

The Portkey API supports creating workspace-level integrations (where the integration is scoped to a specific workspace), but the Terraform provider previously only supported organisation-level integrations.

This PR enables two ways to create workspace-scoped integrations:

1. Using an admin API key with explicit workspace_id:

resource "portkey_integration" "openai_dev" {
  name           = "OpenAI Dev"
  ai_provider_id = "openai"
  key            = var.openai_api_key
  workspace_id   = portkey_workspace.dev.id
}

2. Using a workspace-scoped API key (automatically scoped, no workspace_id needed)

Test plan

• Added acceptance test TestAccIntegrationResource_workspaceScoped
• Verified existing integration tests still pass
• Manually tested via curl that the API correctly creates workspace-scoped integrations

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