Cosmos GraphQL schema should enforce the `id` field be non-null

[aaronpowell]

When creating a GraphQL schema to use with Cosmos, it is possible to define a nullable id field, which will then result in a runtime error trying to use the create mutation, since the id field is required for both the id and partition key.

Our tests are showing this as an incorrect pattern:

type Character @model(name:"Character") {
    id : ID,
    name : String,
    type: String,
    homePlanet: Int,
    primaryFunction: String,
    star: Star
}

type Planet @model(name:"Planet") {
    id : ID,
    name : String,
    character: Character,
    age : Int,
    dimension : String,
    stars: [Star]
    tags: [String!]
}

type Star @model(name:"StarAlias") {
    id : ID,
    name : String
}

There is a test that covers the error you would get at runtime:

data-api-builder/src/Service.Tests/CosmosTests/MutationTests.cs

Lines 148 to 162 in 007d84c

	[TestMethod]
	public async Task MutationMissingRequiredIdReturnError()
	{
	// Run mutation Add planet without id
	const string name = "test_name";
	string mutation = $@"
	mutation {{
	createPlanet (item: {{ name: ""{name}"" }}) {{
	id
	name
	}}
	}}";
	JsonElement response = await ExecuteGraphQLRequestAsync("createPlanet", mutation, variables: new());
	Assert.IsTrue(response[0].GetProperty("message").ToString().Contains("The input content is invalid because the required properties - 'id; ' - are missing"));
	}

Since this is a requirement, it should be made clear in the GraphQL schema that is served by DAB, to ensure that clients are aware of what the contract is with the server, rather than waiting until runtime to receive an error response.

[aaronpowell]

As part of #1235 I'm going to update the GraphQL schema used by the Cosmos tests to have the ID field on Planet non-nullable, and I'll update the test to verify the correct error message, but it won't address the underlying problem