How to use custom HTTP clients with Twilio in Java

A Java developer building an application that uses Twilio's API may want more control over the HTTP client when making API calls. To achieve this, they can use a custom instance of `TwilioRestClient` to configure their HTTP client with specific credentials or proxy settings. For example, using multiple sets of Twilio credentials for different accounts or projects requires creating separate instances of `TwilioRestClient` with different account and auth token combinations. Similarly, when working behind an HTTP proxy, a custom instance of `HttpClient` can be used to configure the proxy settings. By providing these custom clients to the API calls, developers can gain more control over their Twilio API usage and ensure that they are using the correct credentials or proxy settings for their specific use case.