fix: send auth headers with info requests if missing

[kruskall]

Forward auth headers for info requests. This does not affect data ingestion as we are already forwarding the headers there.

A test has been added to ensure the auth options passed to the lambda extension are correctly forwarded.

Closes #389

[lahsivjar]

Tested with a custom go lambda function which makes a call to http://127.0.0.1:8200/ and logs the body. The function was configured to use AWS secrets manager (ref) instead of the secret token directly.

Used go lambda function

package main

import (
	"context"
	"fmt"
	"io"
	"net/http"
	"strconv"
	"strings"

	"github.com/aws/aws-lambda-go/events"
	"github.com/aws/aws-lambda-go/lambda"

	_ "go.elastic.co/apm/module/apmlambda/v2"
)

var coldstart = true

func Handle(ctx context.Context, req events.APIGatewayProxyRequest) (events.APIGatewayProxyResponse, error) {
	var body strings.Builder
	resp, err := http.Get("http://127.0.0.1:8200/")
	if err != nil {
		body.WriteString(fmt.Sprintf("error in info request: %+v", err))
	} else {
		sb, _ := io.ReadAll(resp.Body)
		body.WriteString("body: ")
		body.Write(sb)
		body.WriteString(", ")
		body.WriteString("status_code: ")
		body.WriteString(strconv.Itoa(resp.StatusCode))
		body.WriteString(", ")
		body.WriteString("status: ")
		body.WriteString(resp.Status)
	}
	defer resp.Body.Close()
	response := events.APIGatewayProxyResponse{
		StatusCode: 200,
		Body:       body.String(),
		Headers: map[string]string{
			"coldstart": strconv.FormatBool(coldstart),
		},
	}
	coldstart = false
	return response, nil
}

func main() {
	lambda.Start(Handle)
}

Observed response without this PR

body: , status_code: 200, status: 200 OK

Observed response with this PR

body: {"build_date":"2023-02-13T13:01:54Z","build_sha":"8638b035d700e5e85e376252402b5375e4d4190b","publish_ready":true,"version":"8.6.2"}
, status_code: 200, status: 200 OK