Support control_plane attribute in ske cluster

[GokceGK]

Description

relates to STACKITTPR-517

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[github-actions]

This PR was marked as stale after 7 days of inactivity and will be closed after another 7 days of further inactivity. If this PR should be kept open, just add a comment, remove the stale label or push new commits to it.

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/ske	0.00% (ø)
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/ske/cluster	50.00% (+0.72%)	👍

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/ske/cluster/datasource.go	0.00% (ø)	39	0	39
github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/ske/cluster/resource.go	52.21% (+0.71%)	881 (+17)	460 (+15)	421 (+2)	👍

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/ske/cluster/resource_test.go
• github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/ske/ske_acc_test.go