Allow override of GKE Nodepool SA Display Name

[robrankin]

When creating a custom nodepool SA, the display name can exceed the maximum length allowed of 100.

This PR allows for setting a custom display name on the nodepool SA, but keeps the default construction if display_name is not specified.

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

❯ pytest -k 'gke-nodepool' tests/examples
==== test session starts =====
platform darwin -- Python 3.11.6, pytest-8.3.3, pluggy-1.5.0
rootdir: /Users/robertrankin/Development/Terraform/rr2_cloud-foundation-fabric/tests
configfile: pytest.ini
plugins: xdist-3.6.1
collected 625 items / 619 deselected / 6 selected

tests/examples/test_plan.py ......                                                                                                                                                                                                           [100%]

==== 6 passed, 619 deselected in 118.79s (0:01:58)

[ludoo]

thanks :)