feat: support config::Union type as input type

[meskill]

Proposal

Allow to use Union types both as output and input types.

How to

Union type doesn't have exact representation as input type.

Proposal suggests to represent union type as input types do next:

For every member of union type that are used as field's arg generate new multiple fields in parent object with arguments that use this member of union type. Field names will be different and arguments should not use union type name but name of the inner types. The inner types should be outputed as input types in sdl.

I.e. for

{
	"types": {
		"T1": {},
		"T2": {},
		"V1": {},
		"V2": {},
		"Query": {
			"fields": {
				"call": {
					"type": "String",
					"args": {
						"str": {
							"type": "String"
						},
						"u1": {
							"type": "U1"
						},
						"u2": {
							"type": "U2"
						}
					}
				}
			}
		}
	},
	"unions": {
		"U1": ["T1", "T2"],
		"U2": ["V1", "V2"]
	},
	"schema": {
		"query": "Query"
	}
}

generate following sdl:

input T1 {}
input T2 {}
input V1 {}
input V2 {}

type Query {
	callVar0Var0(str: String, u1: T1, u2: V1): String
	callVar0Var1(str: String, u1: T1, u2: V2): String
	callVar1Var0(str: String, u1: T2, u2: V1): String
	callVar1Var1(str: String, u1: T2, u2: V2): String
}

Another approaches

• @oneof directive from draft spec - but this adds additional nesting for types that should be separated by unions and it's hard to handle it.

[meskill]

Implemented in #1992

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