rename `body` to `data` in request options

[gr2m]

The endpoint() method currently returns body as the key holding the request body. We should rename it to data to align with data parameter in @octokit/request: https://github.com/octokit/request.js/#the-data-parameter--set-request-body-directly

We can start by setting both .data and .body and then logging a deprecation message when .body is accessed.