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-server/src/test/kotlin/org/pkl/server/AbstractServerTest.kt

@@ -1029,7 +1029,8 @@ abstract class AbstractServerTest {
         rootDir = null,
         cacheDir = cacheDir,
         outputFormat = null,
-        project = project
+        project = project,
+        http = null,
       )
 
     send(message)

pkl-server/src/test/kotlin/org/pkl/server/JvmServerTest.kt

@@ -19,7 +19,6 @@ import java.io.PipedInputStream
 import java.io.PipedOutputStream
 import org.junit.jupiter.api.AfterEach
 import org.junit.jupiter.api.BeforeEach
-import org.pkl.core.http.HttpClient
 
 class JvmServerTest : AbstractServerTest() {
   private val transports: Pair<MessageTransport, MessageTransport> = run {
@@ -35,7 +34,7 @@ class JvmServerTest : AbstractServerTest() {
   }
 
   override val client: TestTransport = TestTransport(transports.first)
-  private val server: Server = Server(transports.second, HttpClient.dummyClient())
+  private val server: Server = Server(transports.second)
 
   @BeforeEach
   fun beforeEach() {

pkl-server/src/test/kotlin/org/pkl/server/MessagePackCodecTest.kt

@@ -112,7 +112,8 @@ class MessagePackCodecTest {
                 "baz" to
                   RemoteDependency(URI("package://localhost:0/baz@1.1.0"), Checksums("abc123"))
               )
-          )
+          ),
+        http = null,
       )
     )
   }