Add support for importing role mappings

[tobio]

Fixes #1440

[Copilot]

Pull Request Overview

This PR adds import functionality to the Elasticsearch security role mapping resource, addressing issue #1440. The implementation follows the Terraform provider framework's standard pattern for enabling resource imports.

Key Changes:

• Adds ImportState method to enable importing existing role mappings into Terraform state
• Includes acceptance test to verify import functionality works correctly

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/elasticsearch/security/role_mapping/resource.go	Implements ImportState method and adds interface compliance checks for import support
internal/elasticsearch/security/role_mapping/acc_test.go	Adds import state verification test step to existing acceptance test