Add new schema filter to exclude nullable fields from required

[ivan-luxoft]

Swashbuckle.AspNetCore has a feature NonNullableReferenceTypesAsRequired, but specification has required flag for nullable and non nullable properties and doesn't any difference between them.

for example dto:

public record Dto
{
    public required Dto RequiredNonNull { get; init; }

    public required Dto? RequiredNull { get; init; }

    public Dto NonRequiredNonNull { get; init; } = null!;

    public Dto? NonRequiredNull { get; init; }
}

has component:

{
	"Dto": {
		"required": [
			"nonRequiredNonNull",
			"requiredNonNull",
			"requiredNull"
		],
		"properties": {
			"key": {
				"type": "string"
			},
			"requiredNonNull": {
				"$ref": "#/components/schemas/Dto"
			},
			"requiredNull": {
				"$ref": "#/components/schemas/Dto"
			},
			"nonRequiredNonNull": {
				"$ref": "#/components/schemas/Dto"
			},
			"nonRequiredNull": {
				"$ref": "#/components/schemas/Dto"
			}
		},
		"additionalProperties": false
	}
}

and swashbuckle (openapi?) cant add nullable flag together with $ref, so one of the way to avoid that - exclude nullable fields from required