Validate Inline struct attributes

[johakoch]

"Regular" config struct attributes are validated at config load. Several Inline attributes, however, are not validated at all.

E.g. proxy:

func (p Proxy) Inline() interface{} {
	type Inline struct {
		meta.RequestHeadersAttributes
		meta.ResponseHeadersAttributes
		meta.FormParamsAttributes
		meta.QueryParamsAttributes
		Backend        *Backend    `hcl:"backend,block" ...`
		ExpectedStatus []int       `hcl:"expected_status,optional" ...`
		URL            string      `hcl:"url,optional" ...`
		Websockets     *Websockets `hcl:"websockets,block" ...`
	}

	return &Inline{}
}

---

proxy {
  url = 42
}

URL is validated at runtime by producer.NewURLFromAttribute(). OK

---

proxy {
  expected_status = 200
}

ExpectedStatus with an invalid type (e.g. int instead of []int) is quietly ignored:
handler/proxy.go

	expStatusVal, err := eval.ValueFromBodyAttribute(hclCtx, p.context, "expected_status")
	if err != nil {
		return nil, err
	}

	outCtx = context.WithValue(outCtx, request.EndpointExpectedStatus, seetie.ValueToIntSlice(expStatusVal))

with

func ValueToIntSlice(src cty.Value) []int64 {
	var n []int64
	if !src.IsKnown() || src.IsNull() || !src.CanIterateElements() {
		return n
	}

returning an empty slice, then ignored in handler/producer/result.go because len() of the empty slice is not > 0

	if expStatus, ok := req.Context().Value(request.EndpointExpectedStatus).([]int64); beresp != nil &&
		ok && len(expStatus) > 0 {
...

---

proxy {
  set_request_headers = 42
}

even panics.

---

As it seems, the Inline is only used to create another BodySchema to find unsupported attributes/blocks in (config/configload/schema.go)

func completeSchemaComponents(body hcl.Body, schema *hcl.BodySchema,
	blocks hcl.Blocks, errors hcl.Diagnostics) (hcl.Blocks, hcl.Diagnostics) {

	content, diags := body.Content(schema)

How can we use the type information from the Inline structs returned by .Inline() to validate attribute values?