warn user of incompatible `fetcher` configuration when `buildService` is used

I was trying to configure a custom fetcher for my gateway. I tried to pass in `fetcher` as an argument, but it wasn't getting used. I later [learned](https://github.com/apollographql/federation/blob/732f8638a0ce428c8b0431f299f12f6f80d11a61/docs/source/api/apollo-gateway.mdx#configuring-the-subgraph-fetcher) that if you're specifying a `buildService` call, you need to configure the fetcher in a different way.

A warning that says "Hey, you're specifying a fetcher.. but also a build service. Note that this won't get picked up" would have saved me a handful of hours of debugging.

Start with docs/source/api/apollo-gateway.mdx, especially the configuring-the-subgraph-fetcher section, to understand the interaction between fetcher and buildService. Trace the gateway configuration entry point and existing warning behavior; done means an incompatible fetcher configuration produces a clear warning when buildService is also specified.

Written by the indexing model from the issue text.