Improve backend code coverage performance

[This code](https://github.com/cypress-io/code-coverage/blob/eca3ba7af1a16737e2a06842a395b0afe61b50c0/support.js#L193-L222) uses a `cy.request` to query the running dev servers to get their coverage which then just gets loaded in client-side and immediately sent to the Cypress server via a `cy.task`. It likely could be more efficient (skip one network hop, avoid transferring the data to the browser, and populating the cy.task message field with the large amount of data) to query the dev server directly from the Cypress app server.

The only way this falls apart is if the dev servers have to be accessed via `cy.request` but i'm not sure that is a likely scenario. We could always detect whether the coverage routes are accessible first and then fallback to the existing logic otherwise.