Should the `internal.HTTPClient` retry when Google's servers return 502 default for the Messaging Endpoint?

When Google's servers reponse 502 for the Messaging Endpoint(https://fcm.googleapis.com/v1) requests, the response body has been read in advance and the `RetryConfig` can catch underlying network errors, as the following comment says,

```go
// https://github.com/firebase/firebase-admin-go/blob/master/internal/http_client.go#L194
// ...
// If a RetryConfig is available, always consult it to determine if the request should be retried
// or not. Even if there was a network error, we may not want to retry the request based on the
// RetryConfig that is in effect.
if c.RetryConfig != nil {
delay, retry := c.RetryConfig.retryDelay(retries, resp, result.Err)
result.RetryAfter = delay
result.Retry = retry
}
```
But for the choice of most users, the default retry config `WithDefaultRetryConfig` only retries when `http.StatusServiceUnavailable` **503** occurred.

```go
// https://github.com/firebase/firebase-admin-go/blob/master/internal/http_client.go#L82
func WithDefaultRetryConfig(hc *http.Client) *HTTPClient {
twoMinutes := time.Duration(2) * time.Minute
return &HTTPClient{
Client: hc,
RetryConfig: &RetryConfig{
MaxRetries: 4,
CheckForRetry: retryNetworkAndHTTPErrors(
http.StatusServiceUnavailable,
),
// ...
```

And it is true that Google often returns 502 for the Messaging Endpoint requests as shown below,

```html

Error 502 (Server Error)!!1

*{margin:0;padding:0}html,code{font:15px/22px arial,sans-serif}html{background:#fff;color:#222;padding:15px}body{margin:7% auto 0;max-width:390px;min-height:180px;padding:30px 0 15px}* > body{background:url(//www.google.com/images/errors/robot.png) 100% 5px no-repeat;padding-right:205px}p{margin:11px 0 22px;overflow:hidden}ins{color:#777;text-decoration:none}a img{border:0}@media screen and (max-width:772px){body{background:none;margin-top:0;max-width:none;padding-right:0}}#logo{background:url(//www.google.com/images/branding/googlelogo/1x/googlelogo_color_150x54dp.png) no-repeat;margin-left:-5px}@media only screen and (min-resolution:192dpi){#logo{background:url(//www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png) no-repeat 0% 0%/100% 100%;-moz-border-image:url(//www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png) 0}}@media only screen and (-webkit-min-device-pixel-ratio:2){#logo{background:url(//www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png) no-repeat;-webkit-background-size:100% 100%}}#logo{display:inline-block;height:54px;width:150px}

502. That’s an error.

The server encountered a temporary error and could not complete your request.

Please try again in 30 seconds. That’s all we know.
```