add session affinity values: "GENERATED_COOKIE", "HEADER_FIELD", "HTTP_COOKIE" to variables-backend-service.tf

[tamartayar]

---

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

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[juliocc]

Hi @tamartayar, can you sign the CLA?

[tamartayar]

Hi @tamartayar, can you sign the CLA?

Hi

i signed on the CLA