Support V2 payloads for Echo

[AlexLast]

Issue #, if available:

Description of changes:
Adds support for API Gateway HTTP V2 payloads for the Echo framework

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[speza]

I think it would be better to separate V2 like the Gin adapter (since that is already in) - https://github.com/awslabs/aws-lambda-go-api-proxy/tree/master/gin

[AlexLast]

@speza I used #100 as an example, which was also merged. But happy to refactor if that is the preferred way of adding V2 support?

[speza]

Oh, I didn't see that one! I think my personal preference would be separate. But it appears there is not any consistency!

[AlexLast]

Seeing as Gin is one of the main framework examples for this project I think it makes sense to follow the same pattern, will refactor 👍

[AlexLast]

Have refactored to match the pattern set in the Gin adapter

[yasuoza]

Any update for this PR?
This works fine for me.

[AlexLast]

Any update for this PR? This works fine for me.

@yasuoza Unfortunately It's still waiting for a review, currently I'm just using my own build

[yasuoza]

FYI: Bypass to echo.ServeHTTP also works. This may be useful until this PR is merged.

package main

import (
	"context"
	"log"
	"net/http"

	"github.com/aws/aws-lambda-go/events"
	"github.com/aws/aws-lambda-go/lambda"
	"github.com/awslabs/aws-lambda-go-api-proxy/handlerfunc"
	"github.com/labstack/echo/v4"
)

var (
	httpLambda *handlerfunc.HandlerFuncAdapterV2
)

func init() {
	// stdout and stderr are sent to AWS CloudWatch Logs
	log.Printf("echo cold start")
	e := echo.New()
	e.GET("/", func(c echo.Context) error {
		return c.JSON(http.StatusOK, map[string]string{"message": "pong"})
	})

	httpLambda = handlerfunc.NewV2(e.ServeHTTP)
}

// Handler handles request from API Gateway V2
func Handler(ctx context.Context, req events.APIGatewayV2HTTPRequest) (events.APIGatewayV2HTTPResponse, error) {
	return httpLambda.ProxyWithContext(ctx, req)
}

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