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 4d5a06a contains only introduction of generic CRUD entities (inner and outer)
• Commit dc93038 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 bfabfb0)

[dataclouder]

First pass, mainly dealing with documentation and comments. More after testing

[lvirbalas]

Thanks for a huge refactoring and reduction of repeated code. Let's test the nuts of it before merging!

[adambarreiro]

Running some tests with CSE, LGTM, will approve afterwards