[FAST] Fix IAM bindings to impersonate resman CICD SAs at bootstrap stage

[jayBana]

Resolved an issue that appeared when applying FAST's bootstrap stage where merging blocks with conflicting key names were utilised to apply IAM bindings for service account impersonation between service accounts involved in CICD workload authorisation. This conflict resulted in only the final IAM binding from the merge block being applied, leading to incomplete and therefore incorrect IAM configurations that manifested itself during deployments using GitHub Actions.

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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

[ludoo]

Thanks a lot Janos!