Add support for target_resources to net-firewall-policy module

[bcorbitt-ps]

Add support for target_resources in hierarchical policy for net-firewall-policy module. (Issue #2011)

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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[ludoo]

Thanks a lot for implementing this!