feat: Add NullableFromJSONSchemaTags option

[candiduslynx]

This changes default behavior to look for the jsonschema:"nullable" tag to mark field optional only Reflector.NullableFromJSONSchemaTags is set to true.

In Go it's perfectly fine to unmarshal the following types from null:

• pointers
• slices
• maps
• interfaces

This PR also includes the changes from #107, so it might be better to merge that one first.

[candiduslynx]

@samlown this changes the default behavior to be closer to default encoding/json implementation.

The other option could be make the default behavior to look for tags & only explicitly use field type for nullability (when some option is set).

[samlown]

@candiduslynx thanks for this, but I'm trying to understand what exact problem you're trying to solve with this PR. For me, the nullable type is implied when there is no value, I'm really not sure why you'd want to define it explicitly.

[candiduslynx]

@samlown consider json {"a":null}.
If schema defines property a having "type": "object", the validation will fail.
But the {} might validate successfully, if a isn't a required property.
So, for structs that don't have json:",omitempty" a perfectly valid marshaled representation with explicit null value will fail.

Consider the example added in 26eed64:

type TestNullableField struct {
	A *string `json:"a"`
}

Old behavior (`NullableFromJSONSchemaTags:true`)

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$id": "https://github.com/invopop/jsonschema/test-nullable-field",
  "$ref": "#/$defs/TestNullableField",
  "$defs": {
    "TestNullableField": {
      "properties": {
        "a": {
          "type": "string"
        }
      },
      "additionalProperties": false,
      "type": "object",
      "required": [
        "a"
      ]
    }
  }
}

New behavior (`NullableFromJSONSchemaTags:false`)

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$id": "https://github.com/invopop/jsonschema/test-nullable-field",
  "$ref": "#/$defs/TestNullableField",
  "$defs": {
    "TestNullableField": {
      "properties": {
        "a": {
          "oneOf": [
            {
              "type": "string"
            },
            {
              "type": "null"
            }
          ]
        }
      },
      "additionalProperties": false,
      "type": "object",
      "required": [
        "a"
      ]
    }
  }
}

In both cases a is required, but the old behavior will fail for {"a":null} whereas the new one will succeed.

[candiduslynx]

@samlown This PR also includes the changes from #107, so it might be better to merge that one first.

[candiduslynx]

I'm going to reopen a fresh version that preserves default behavior + add README.md doc for this

[candiduslynx]

Replaced by #110