chore: expose internal RequestClient to users at top level

[stern-shawn]

Fixes

Exposes the internal RequestClient so users don't need to import relying on twilio-node's file structure, ex: const RequestClient = require('twilio/lib/base/RequestClient'); and can instead import via:

const twilio = require('twilio');

const { RequestClient } = twilio;

The use case is when users are writing some type of custom client but they don't want to directly access axios, etc and instead want to lean on Twilio's existing client, for example:

const twilioClient = twilio(accountSid, authToken, {
  httpClient: new CustomRedirectClient('http://localhost:8080/mock-server', new RequestClient()),
});

Adds RequestClient to typedefs.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• [n/a] I have added tests that prove my fix is effective or that my feature works
• [n/a] I have added the necessary documentation about the functionality in the appropriate .md file
• [n/a] I have added inline documentation to the code I modified

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