[Next.js] Support setting different options for different runtimes (browser/server/edge)

Originally posted by @ricardomatias [in discussions](https://github.com/getsentry/sentry-javascript/discussions/13833):

> I've removed the sentry.edge.config.ts and sentry.server.config.ts and I still get sourcemaps for both of them. I only want client sourcemaps.

I don't think this is possible right now without some very ugly hacks. We should provide some way of setting different options for the different runtimes.

Idea No.1 and probably my favorite: Allow for the `sentryBuildOptions` argument in `withSentryConfig` to be a function/callback that receives the runtime as an argument. (probably should pass it within an object so that we can extend it in the future)