Fine-grained nullish inference

[seiyab]

Is your feature request related to a problem? Please describe.
I want to control whether a slice field is nullable or not. JSONSCHEMAGODEBUG environment variable might help but I still feel inconvenient:

• Not build-time configuration
  • It can cause mismatch between dev and prod
• Less configurable
  • It is all or nothing. Can't control for each schema.

Describe the solution you'd like

// Approach 1: tag
type A struct {
  Things []int `jsonschemanull:"false"`
}

// Approach 2: option
For[MyType](&ForOption{
  Null: true, // we should consider more understandable name
})

Describe alternatives you've considered
N/A

Additional context
I want to utilze it to generate schema and pass it to a library. But the library we use doesn't support ["null", "array"] as of now.