Add IsRequired validators to list, set, and object validators for Blocks

[austinvalle]

closes #105

• This PR introduces 3 new validators that will provide equivalent Required functionality to:
  • ListNestedBlock
  • SetNestedBlock
  • SingleNestedBlock

Example validation error:

Error: Invalid Block

	with data.cloudinit_config.foo,
	on terraform_plugin_test.tf line 1, in data "cloudinit_config" "foo":
	 1: data "cloudinit_config" "foo" {
	
Block part must have a configuration value set as the provider has marked
it as required

https://pkg.go.dev/github.com/hashicorp/terraform-plugin-framework-validators@v0.9.1-0.20230206221608-25bada52026a

Notes

• All validators referenced Attributes in the error messages, and I updated mine to Block. Not sure if this will cause unnecessary confusion?
• I originally had a comment noting objectvalidator.IsRequired usage with schema.NestedBlockObject, but looking at it now, it doesn't seem like that would be useful?

[bendbennett]

LGTM 👍

Do we need to give any consideration to the possibility of these validators being used erroneously? For instance:

func (e *exampleResource) Schema(ctx context.Context, req resource.SchemaRequest, resp *resource.SchemaResponse) {
	resp.Schema = schema.Schema{
		Attributes: map[string]schema.Attribute{
			"set_nested_attribute": schema.SetNestedAttribute{
				Optional: true,
				Validators: []validator.Set{
					setvalidator.IsRequired(),
				},
			},
		},
	}
}

[austinvalle]

LGTM 👍

Do we need to give any consideration to the possibility of these validators being used erroneously? For instance:

func (e *exampleResource) Schema(ctx context.Context, req resource.SchemaRequest, resp *resource.SchemaResponse) {
	resp.Schema = schema.Schema{
		Attributes: map[string]schema.Attribute{
			"set_nested_attribute": schema.SetNestedAttribute{
				Optional: true,
				Validators: []validator.Set{
					setvalidator.IsRequired(),
				},
			},
		},
	}
}

That's a good question. I think we already have the potential of validators conflicting that we don't prevent:

"example_string": schema.StringAttribute{
	Validators: []validator.String{
		stringvalidator.LengthAtLeast(5),
		stringvalidator.LengthAtMost(2),
	},
},

And as you noted, this PR introduces conflicts between our "built-in" fields Required + Optional. I'd think as long as the behavior is predictable to a provider developer it shouldn't be too much of a concern, but I could be overlooking something.

As well as this validation is a little special, as it's meant only for the Block types, which don't have Required / Optional, but it can still be used anywhere regardless 🤔

[austinvalle]

Bubbled up conversation to our slack, I think this is good to merge for now

[bflad]

Added labels and assigned to upcoming milestone.

[bflad]

FYI, this also needs changelog entries: https://github.com/hashicorp/terraform-plugin-framework-validators/blob/main/.github/CONTRIBUTING.md#changelog

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