Fix up nullability and primitive type handing in schema generation

[captainsafia]

• Use IsGetNullable and IsSetNullable properties to set nullability status for properties
• Use per-instance NullabilityInfoContext instead of shared static instance
• Avoid allocating schemas for select primitive types
• Avoid including special ? character in reference ID for nullable schemas
• Massage representation of object schemas to close object property causes build crash when generating OpenAPI document at build time #56351

Addresses feedback from #56330 (comment) and #56318

[BrennanConroy]

Are these from an OpenAPI spec? Otherwise there are probably other primitives, e.g. nint, byte

[captainsafia]

Added these and a few more other primitive types. Lemme know what you think of the list.

[martincostello]

Any of Guid, Uri, TimeSpan, DateOnly and TimeOnly? Just comparing to the list we have in Swashbuckle and they're the only ones that are different to this set. They basically end up as a string with a format of some kind in the schema.

[captainsafia]

Ah, yes! I recall you sharing this in the other PR. I've updated this set so it's consistent between the two. I wasn't able to find the logic in NSwag for this but this probably a set that we should be consist with throughout the implementations. 🤔

[martincostello]

I'd forgotten about Version 😄

Did you want TimeSpan too, or was there a reason to skip that?

[captainsafia]

Yet another reason to make this public 😓 Added that as well. Let me know if there is anything else I missed. I'd like to get this PR in soon to ease some of the build failures it is causing but happy to add things in a follow-up PR.

[martincostello]

The new set seems to cover all the common primitives/cases I'm aware of 👍