Add support for HTTP proxying (#506)

* 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

pkl-executor/src/test/kotlin/org/pkl/executor/EmbeddedExecutorTest.kt

@@ -41,7 +41,7 @@ class EmbeddedExecutorTest {
     }
     override fun toString(): String = name
   }
-  
+
   companion object {
     @JvmStatic
     private val allTestExecutors: List<TestExecutor> by lazy {