Add support for Provider VDCs read functions and upgrade Compute Policies version

[adambarreiro]

Description

This PR sets the foundations required for https://github.com/vmware/terraform-provider-vcd/pull/904:

• Adds retrieval support for Provider VDCs, both "regular" and the "extension". The extension is a superset of attributes not present in "regular" Provider VDC endpoints. In VCD UI, both endpoints are used.
• Updates VDC Compute Policies retrieval methods AdminVdc.GetAllAssignedVdcComputePolicies and Org.GetAllVdcComputePolicies from OpenAPI v1.0.0 to v2.0.0, this version supports more filtering options like isVgpuPolicy
• Adds some utility private functions and fixes some typos.

[adambarreiro]

I removed Vdcs slice from ProviderVdc struct as I confirmed that this field is never populated nor used.