Add Webauthn policy to Keycloak

Currently, the Webauthn policy gets overridden on every single terraform apply which sucks a lot!

We urgently need to manage this via Terraform properly. It needs these PRs to be merged prior:

• https://github.com/mrparkers/terraform-provider-keycloak/pull/356
• https://github.com/mrparkers/terraform-provider-keycloak/pull/357
• https://github.com/mrparkers/terraform-provider-keycloak/pull/385 (not as urgent as 356)

@klausenbusk Could you take a look at your PRs over at the provider's side?