直接上代码，内容慢慢补

package main

import (
	"context"
	"crypto/rsa"
	"crypto/x509"
	"encoding/pem"
	"net/http"
	"os"
	"time"

	"github.com/wechatpay-apiv3/wechatpay-go/core"
)

var (
	client *core.Client
	ctx    context.Context
)

func init() {
	var err error
	var (
		mchID                      string              = "1234567890"                               // 商户号
		mchCertificateSerialNumber string              = "4F00000000000000000000000000000000000021" // 商户API证书序列号
		mchPrivateKey              *rsa.PrivateKey                                                  // 商户API私钥
		wechatPayCertList          []*x509.Certificate                                              // 微信支付平台证书
		customHTTPClient           *http.Client
		customHTTPHeader           http.Header
	)

	keyPEM, err := os.ReadFile("apiclient_key.pem")
	if err != nil {
		panic(err)
	}
	block, _ := pem.Decode([]byte(keyPEM))
	if block == nil {
		panic("failed to parse privateKey PEM")
	}

	privateKey, err := x509.ParsePKCS8PrivateKey(block.Bytes)
	if err != nil {
		panic(err)
	}

	mchPrivateKey = privateKey.(*rsa.PrivateKey)

	certPEM, err := os.ReadFile("apiclient_cert.pem")
	if err != nil {
		panic(err)
	}
	block, _ = pem.Decode([]byte(certPEM))
	if block == nil {
		panic("failed to parse certificate PEM")
	}
	cert, err := x509.ParseCertificate(block.Bytes)
	if err != nil {
		panic(err)
	}
	wechatPayCertList = []*x509.Certificate{cert}

	ctx = context.Background()
	opts := []core.ClientOption{
		core.WithMerchantCredential(mchID, mchCertificateSerialNumber, mchPrivateKey), // 必要，使用商户信息生成默认 WechatPayCredential
		core.WithWechatPayValidator(wechatPayCertList),                                // 必要，使用微信支付平台证书列表生成默认 WechatPayValidator
		core.WithHTTPClient(customHTTPClient),                                         // 可选，设置自定义 HTTPClient 实例，不设置时使用默认 http.Client{}
		core.WithTimeout(2 * time.Second),                                             // 可选，设置自定义超时时间，不设置时使用 http.Client{} 默认超时
		core.WithHeader(customHTTPHeader),                                             // 可选，设置自定义请求头
	}
	client, err = core.NewClient(ctx, opts...)
	if err != nil {
		panic(err)
	}
}

Q.E.D.