add ability to unset policy tags (and description?) in schema fields

[tswast]

Is your feature request related to a problem? Please describe.

I noticed in #557 that we don't send the policy tags property if none are set in SchemaField. I believe this tells the backend not to make any updates to the policy tags property. This means it's not possible to unset policy tags from Python.

(Note: I haven't verified this, so some system tests / samples should be written for this use case)

Describe the solution you'd like

Similar to what I did for description in #557, we should disambiguate between unset policy tags and empty / none for policy tags.

Describe alternatives you've considered

We could make the update fields property more fine-grained, such as schema.policytags in order to force a sub-property to be included. This feels overly complicated, though.

Additional context

N/A

[tswast]

Since SchemaField is read-only, we could also make the change to use _properties for all properties without making breaking changes, which would align it with our other resource classes.

[plamut]

I noticed in #557 that we don't send the policy tags property if none are set in SchemaField. I believe this tells the backend not to make any updates to the policy tags property. This means it's not possible to unset policy tags from Python.

(Note: I haven't verified this, so some system tests / samples should be written for this use case)

I tried it and if schema_field.policy_tags is set from "something" to None, it is simply omitted from the API representation, meaning that policy tags indeed not get unset.