[release/v15] [Feature] Add user principal id to global role binding

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Backport #2153 to release/v15 for #2198

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Summary

• Add user_principal_id field to rancher2_global_role_binding resource, enabling assignment of global roles to remote users via their principal ID
• Follows the same pattern already used in rancher2_cluster_role_template_binding and rancher2_project_role_template_binding
• Adds user_principal_id to the data source as well for completeness

Related

Fixes #2152

Changes

File	Change
rancher2/schema_global_role_binding.go	Added user_principal_id field to schema
rancher2/structure_global_role_binding.go	Added flatten/expand support for UserPrincipalID
rancher2/data_source_rancher2_global_role_binding.go	Added user_principal_id computed attribute
rancher2/structure_global_role_binding_test.go	Added test coverage for user_principal_id
docs/resources/global_role_binding.md	Added argument reference, example, and note
docs/data-sources/global_role_binding.md	Added attribute reference

Testing

• TestFlattenGlobalRoleBinding — PASS
• TestExpandGlobalRoleBinding — PASS
• go build ./... — compiles cleanly

Breaking Change Notice

This is a breaking change for existing rancher2_global_role_binding resources.

The user_principal_id field uses ForceNew: true, which means that on upgrade, Terraform will detect a diff between the config (which does not specify user_principal_id) and the state (which will be populated from the Rancher API during refresh). This triggers a destroy and recreate of all existing global role bindings managed by Terraform.

This matches the pattern used in rancher2_cluster_role_template_binding and rancher2_project_role_template_binding, where ForceNew: true was eventually adopted for identity-related fields. The field is immutable at the Rancher API level — a global role binding's principal cannot be changed without recreating the binding.

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