Implement Policy Fetcher implementation for WafPolicy

[ciarams87]

As a user
I want NGF to fetch my NAP WAF Policy bundle from a remote location
So that this bundle can be applied by the data plane to protect my traffic

Acceptance

• When a user supplies a remote location for a WAF policy in the WafPolicy resource:
  • The NGF control plane attempts to download the WAF policy from the remote location supplied.
  • The file downloaded from the remote location is validated via checksum that was downloaded from the remote location.

Dev Notes

• Create a Policy Fetcher utility in framework which can pull files from a remote location, as configured in the WafPolicy resource.
• Implement policy validation via Checksum validation (compute checksum locally and compare to remote provided checksum).