Add functions for generic OpenAPI CRUD handling

[Didainius]

About

This PR is about an attempt to leverage Go generics (parametric polymorphism) to streamline SDK development. As part of this PR, a few of the existing entities were converted to demonstrate the use of new functions.

Terminology

These names are not set in stone and are subject to change.

• inner
• outer*

Commits

• Commit https://github.com/vmware/go-vcloud-director/pull/644/commits/4d5a06a46611a22fecd7403a932edd85aedfed52 contains only introduction of generic CRUD entities (inner and outer)
• Commit https://github.com/vmware/go-vcloud-director/pull/644/commits/dc93038aed613c5fbcd30dbf55352ece63c05dea adds consumption examples throughout our codebase

Next PRs contain self review and fixed over all codebase.

Testing

• Test suite passed on 10.5.1 for both SDK and Terraform (at commit https://github.com/vmware/go-vcloud-director/pull/644/commits/bfabfb0a024ad4875c1170404ad5af708b43e4db)