[RFE] Improve Auth Management

[mitchwadair]

Is your feature request related to a problem? Please describe.
Currently every request to Twitch made be TES validates the access token first. This could cause some rate-limiting issues (maybe, not sure how those are handled) for users of the library that have large loads of users or something that would cause them to need to make those calls. It also makes every operation requiring the API very inefficient, especially those using pagination.

Describe the solution you'd like
An auth service type thing that manages the app access token and refreshing it. This should also mean that should a request be made where an invalid token is used (is possible, and could also happen with some extremely rare timing), that it is handled as well.

The auth service should (for app access tokens, i.e. for webhooks users), should store the current token and refresh token and schedule token refresh. It should also periodically verify the token. Periodic verification should either happen on some kind of timer, or maybe randomly before some requests are made. A good way to maybe do this is have some kind of request manager that would select some requests to verify every so often

Some possible examples of usage:

import AuthService from "authservicepath";

const token  = AuthService.getToken();

fetch("someurl", {method: "GET", headers: { "Authorization": `Bearer ${token}` }})
    .then(res => res.json())
    .then(data => {
        // do things
    })
    .catch(err => {
        if (err.message === "token not valid") {
            AuthService.refreshToken();
            // somehow make request again
            // maybe this would be a good thing for a request manager to deal with??
        }
    });