Support Policy Definition Schema

**Is your feature request related to a problem? Please describe.**
Currently the `policyRule` property on `microsoft.authorization/policydefinitions` is typed as `any`, however it requires a very specific object schema. It would be nice if the type definition could be a bit stronger

**Describe the solution you'd like**
There does seem to be a schema defined for hte policy rule object:
https://github.com/Azure/azure-resource-manager-schemas/blob/main/schemas/2020-09-01/policyDefinition.json

I imagine it should be possible to work that into the openapi definition
https://github.com/Azure/azure-resource-manager-schemas/blob/fa29c2c8ad02412f5ed437d7664a7cfe9bf67c72/schemas/2021-06-01/Microsoft.Authorization.Resources.json#L508-L512

Start by comparing the policy rule schema in schemas/2020-09-01/policyDefinition.json with the policyRule definition in schemas/2021-06-01/Microsoft.Authorization.Resources.json at lines 508-512. Trace how that OpenAPI definition becomes the Bicep type for microsoft.authorization/policydefinitions. Done means policyRule is represented by the specific schema rather than any.

Written by the indexing model from the issue text.