Add CorsOptions support to https.onCall in v2 functions

In v2, `cors` param on `https.onCall` should accept `cors.CorsOptions` object like on `v1`, the frontend app keeps sending check for CORS (OPTIONS) before every request because there isn't no maxAge set.
I can make a pull request for this, let me know what you think.

Start at the v2 https.onCall entry point and compare its cors handling with the v1 behavior described in the issue. Trace how CorsOptions and maxAge are currently handled, then verify that an options object is accepted and that the resulting callable requests no longer require a CORS preflight on every request.

Written by the indexing model from the issue text.