Gateway: Set `User-Agent` when talking to downstream services.

It seems that, by default, it would be reasonable for `@apollo/gateway` to set a `User-agent` header when talking to downstream services which indicates that its name and version along with some other `process` details like Platform. (e.g., `Apollo Gateway/0.16.7 Node.js/12.x.x Linux/??`).

## TODO

- [ ] Determine / propose exactly which parameters to include prior to opening a PR. 😉
- [ ] While Apollo Gateway doesn't automatically pass headers to downstream services, anyone who did implement that themselves would risk squashing this header. In that implementation, so long as _that other_ header survives, that seems reasonable.

The issue names no source files, tests, or entry points. First clarify which User-Agent parameters and header-preservation behavior are required before locating the gateway's downstream request path. Done means the parameter set and expected behavior are agreed and covered by the relevant tests.

Written by the indexing model from the issue text.