【proxy】RestTemplate自定义设置代理支持

**描述问题**
问题原因是，生产环境部分内网机器，由于安全限制，不能直接访问公网环境，通常通过能够访问公网的机器代理出去访问，如http_prosy,https_proxy，在Java程序中也是如此，通过配置如下，以实现能够访问微信接口
```java
HttpHost proxy = new HttpHost("192.xxx.xxx.xxx", 31280);
CloseableHttpClient httpClient = HttpClientBuilder.create().setProxy(proxy).build();
```

**环境版本信息**
JDK1.8，使用版本:1.0.19.RELEASE

**期望的结果**

此处构建new WechatPayClient()时希望能够支持自定义RestTemplate()
希望增加如下代码做兼容
```java
public WechatPayClient(SignatureProvider signatureProvider, RestTemplate restTemplate) {
this.signatureProvider = signatureProvider;
this.restOperations = restTemplate;
}
```

Start at the WechatPayClient construction path and inspect how its RestTemplate or restOperations is initialized. Add support for supplying a custom RestTemplate, then verify that requests use it and that the existing constructor behavior remains unchanged; the issue does not mention a test file.

Written by the indexing model from the issue text.