message: Adding `customHosts` change
diff: ### Added `browser` field in package.json.
  - The `browser` field indicates the entry point for client applications using Microsoft Graph JS SDK library.
  +### Added `customHosts` options
  +- The `AuthenticationHandler` and `TelemetryHandler` will add or update the request headers only if the request URL is a Graph endpoint or a custom endpoint.
  +- `customHosts` property in `IOptions`, `IClientOptions` takes in a `Set` of custom hosts.
  +
  +```typescript
  +// Consider the custom request url is https://CUSTOM_HOST.com
  +const customHost = "CUSTOM_HOST";
  +const customHosts = new Set<string>([customHost]);
  +const client = Client.initWithMiddleware({ middleware, customHosts });
  +```
  +
  ### Added `GraphClientError`
  - `GraphClientError` handles client-side errors encountered within the JavaScript Client SDK whereas, `GraphError` class should be used to handle errors in the response from the Graph API.
  