fix: make schemas consistent with API

[phm07]

In some places the schema definitions are not consistent with the API documentation, for example properties may not have pointer receivers even though they are marked as optional in the documentation. This results in inconsistent JSON when converting JSON -> Schema -> JSON (as seen for example in hetznercloud/cli#461, where one output is the exact API response and one is serialized and then converted back to JSON).

This PR aims to fix this issue by making the schema definitions consistent.

[jooola]

Isn't this a breaking change ? Or at least a feature?

[phm07]

The schema package is not guaranteed to be consistent between versions as it's intended for internal use

[jooola]

Alright, do we use it externally? Or could we consider moving it into a internal/schema package?

[phm07]

We use them in hcloud/schema.go which is exported, so I don't think we can move them into an internal package

[apricote]

We use it externally to implement the JSON output, as only the schema types contain the JSON tags

[apricote]

I did a search for the import path, and it looks like its only used in hcloud-go, cli and the test suites for HCCM, packer and cluster-api-provider-hetzner: https://sourcegraph.com/search?q=context:global+%22github.com/hetznercloud/hcloud-go/v2/hcloud/schema%22+-file:vendor/*+-repo:github.com/hetznercloud/hcloud-go+-repo:github.com/hetznercloud/cli&patternType=standard&sm=1&groupBy=repo

[phm07]

I made it a fix so it shows up in the changelog. I think that should be enough