Rationale: "proxy" can mean very different things (e.g. java.lang.reflect.Proxy in Java).
This makes the flag name more specific.
CLI:
* `--proxy` -> `--http-proxy`
* `--no-proxy` -> `--http-no-proxy`
Gradle:
* `proxyAddress` -> `httpProxy`
* `noProxy` -> `httpNoProxy`
* Add `--proxy` and `--no-proxy` CLI flags
* Add property `http` to `pkl:settings`
* Move `EvaluatorSettings` from `pkl:Project` to its own module and add property `http`
* Add support for proxying in server mode, and through Gradle
* Add `setProxy()` to `HttpClient`
* Add documentation