hclext: Add schema mode to BodySchema

[wata727]

This PR introduces a "schema mode" to hclext.BodySchema.

Previously, the hclext.Content was not possible to retrieve the body if the nested block contained only attributes with unknown schemas.

body := toHCLBody(`
locals {
  foo = "foo"
  bar = "bar"
}`)

hclext.Content(body, &hclext.BodySchema{
  Blocks: Blocks{
    {
      Type: "locals",
      Body: &hclext.BodySchema{}, // what attributes are declared?
    },
  },
})

On the other hand, in HCL you can retrieve them using JustAttributes.

nestedBody := body.Content(&hcl.BodySchema{
  Blocks: Blocks{
    {
      Type: "locals",
    },
  },
})

nestedBody.JustAttributes() // => "foo", "bar" attributes

This is clearly a problem with supporting nested schema. Here we introduce the concept of modes into the schema, allowing us to call JustAttributes for nested blocks.

body := hclext.Content(body, &hclext.BodySchema{
  Blocks: Blocks{
    {
      Type: "locals",
      Body: &hclext.BodySchema{Mode: hclext.SchemaJustAttributesMode},
    },
  },
})

body.Blocks[0].Body.Attributes  // => "foo", "bar" attributes

Note that in just attributes mode, any other Attributes or Blocks schema are ignored. Internally it calls HCL's JustAttributes, so it returns a diagnostic if the body has blocks. In this mode, there is no difference in behavior between Content and PartialContent because it is schemaless.

This feature is available since TFLint v0.42