Fix: Lambda json parsing

[ben-taussig-solo]

Description

Summary of bug

• The bug occurs when unwrapAsApiGateway is enabled in the lambda filter
  • in this case, we attempt to convert a json payload returned by the upstream lambda into a response forwarded to the downstream client
• in this case, we assumed that we'd receive a payload w/ the form:

{
  "multiValueHeaders": {
    "x-header-foo": [
      "bar1",
       "bar2"
    ]
  }
}

• instead, we choked on a payload like this (value inside inner array has unexpected type)

{
  "multiValueHeaders": [
    "x-header-foo": [
      null
    ]
  ]
  
}

Summary of changes

Primary fix

• when iterating over the header values for a given key in the multiValueHeaders object, we now explicitly confirm that the value is a string before assigning it to a variable of type std::string

- const auto& header_value = inner_it.value().get<std::string>();
+ std::string header_value;
+ if (inner_it.value().is_string()) {
+   header_value = inner_it.value().get<std::string>();
+ } else {
+   header_value = inner_it.value().dump();
+ }

related fix

• this PR also validates that the value pointed to by a given header name in the multiValueHeaders object is an array before attempting to iterate over it
• specifically, we reject the request if the value is of type object
• singleton types are processed without issue, so we do not need to reject requests which contain these types

// need to validate that header_values is an array/iterable
if (!header_values.is_array()) {
  // if it's an object, we reject the request
  if (header_values.is_object()) {
    ENVOY_STREAM_LOG(debug, "invalid multi header value object", stream_filter_callbacks);
    ApiGatewayError error = {500, "500", "invalid multi header value object"};
    return ApiGatewayTransformer::format_error(*response_headers, body, error, stream_filter_callbacks);
  }

  ENVOY_STREAM_LOG(debug, "warning: using non-array value for multi header value", stream_filter_callbacks);
}

status code type checking

• this PR also adds some additional type checking around the status field object
• in particular, we remove an unecessary try/catch block, and get stricter about negative/float/non-integer status code values

- try {
-   status_value = json_body["statusCode"].get<uint64_t>();
- } catch (std::exception& exception){
-   ENVOY_STREAM_LOG(debug, "Error parsing statusCode: " + std::string(exception.what()), stream_filter_callbacks);
-   ApiGatewayError error = {500, "500", "Non-integer status code"};
+ if (!json_body["statusCode"].is_number_unsigned()) {
+   // add duplicate log line to not break tests for now
+   ENVOY_STREAM_LOG(debug, "cannot parse non unsigned integer status code", stream_filter_callbacks);
+   ENVOY_STREAM_LOG(debug, "received status code with value: " + json_body["statusCode"].dump(), stream_filter_callbacks);
+   ApiGatewayError error = {500, "500", "cannot parse non unsigned integer status code"};
  return ApiGatewayTransformer::format_error(*response_headers, body, error, stream_filter_callbacks);
}
+ status_value = json_body["statusCode"].get<uint64_t>();

[ben-taussig-solo]

removed (along with the changes to the BUILD file) to improve compilation times for these tests

on my machine, this reduced the time to compile small changes to the already-cached suite from 80s to 60s

[solo-changelog-bot]

Issues linked to changelog:
solo-io/gloo#8867

[ashishb-solo]

nit: would be nice to move "test-multi-header" to an external const value

[ashishb-solo]

nit: would prefer to use std::optional instead of empty strings as sentinel values

[ashishb-solo]

nit: would prefer to see these as external const values