feat: add required field validation

[ctreatma]

This adds required field validation analogous to the required field validation that exists in the Java generator (TODO: link to example in metal-java).

Required fields are validated by:

1. Creating a list of required property names
2. Unmarshalling the current object into a map with string keys
3. Validating that required property names appear as keys in the unmarshalled map

Templates are updated here: b5c00de
Code is regenerated here: 3e16fd3

[ctreatma]

Managing our own templates feels like too much, but I at least needed to do this in order to poke at options for validating the presence of required fields. Ideally this would be adopted upstream, but we may have to use custom templates at least for a little while.

It seems like we have to provide the full set of templates in order to modify just one or two, but I haven't confirmed that myself yet. We could also potentially keep the templates out of git (pull them out during CI, apply patches, and then generate code) but I feel like that's overly complicated.

[ctreatma]

It seems like we have to provide the full set of templates in order to modify just one or two, but I haven't confirmed that myself yet. We could also potentially keep the templates out of git (pull them out during CI, apply patches, and then generate code) but I feel like that's overly complicated.

I was wrong about this; we can provide a subset of the templates. This PR has been updated to only include the templates that we are customizing in order to check for required fields. It's still a good practice to contribute template changes upstream whenever possible to avoid drift, though; custom templates mean we will miss out on upstream template changes unless we periodically re-sync and/or add yet another complicated patching process.

[displague]

We could also potentially keep the templates out of git (pull them out during CI, apply patches, and then generate code) but I feel like that's overly complicated.

That's what I was going to suggest. It's no more complicated that the existing patching and it makes it clear what changes we are making to the templates. It also makes it clear when we need to reconsider those changes in light up new upstream default templates which CI would stamp out before patching, as you suggest.

[ctreatma]

This is being implemented upstream in OpenAPITools/openapi-generator#16863

[ctreatma]

This PR upgrades openapi-generator in order to get required field validation from upstream: #172