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

docs/modules/bindings-specification/pages/message-passing-api.adoc

@@ -121,6 +121,9 @@ outputFormat: String?
 /// The project dependency settings.
 project: Project?
 
+/// Configuration of outgoing HTTP requests.
+http: Http?
+
 class ClientResourceReader {
   /// The URI scheme this reader is responsible for reading.
   scheme: String
@@ -175,6 +178,54 @@ class Project {
   dependencies: Mapping<String, Project|RemoteDependency>
 }
 
+/// Settings that control how Pkl talks to HTTP(S) servers.
+class Http {
+  /// Configuration of the HTTP proxy to use.
+  ///
+  /// If [null], uses the operating system's proxy configuration.
+  proxy: Proxy?
+}
+
+/// Settings that control how Pkl talks to HTTP proxies.
+class Proxy {
+  /// The proxy to use for HTTP(S) connections.
+  ///
+  /// At the moment, only HTTP proxies are supported.
+  ///
+  /// Example:
+  /// ```
+  /// address = "http://my.proxy.example.com:5080"
+  /// ```
+  address: Uri(startsWith("http://"))?
+
+  /// Hosts to which all connections should bypass a proxy.
+  ///
+  /// Values can be either hostnames, or IP addresses.
+  /// IP addresses can optionally be provided using [CIDR notation](https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing#CIDR_notation).
+  ///
+  /// The only wildcard is `"*"`, which disables all proxying.
+  ///
+  /// A hostname matches all subdomains.
+  /// For example, `example.com` matches `foo.example.com`, but not `fooexample.com`.
+  /// A hostname that is prefixed with a dot matches the hostname itself,
+  /// so `.example.com` matches `example.com`.
+  ///
+  /// Optionally, a port can be specified.
+  /// If a port is omitted, all ports are matched.
+  ///
+  /// Example:
+  ///
+  /// ```
+  /// noProxy {
+  ///   "127.0.0.1"
+  ///   "169.254.0.0/16"
+  ///   "example.com"
+  ///   "localhost:5050"
+  /// }
+  /// ```
+  noProxy: Listing<String>(isDistinct)
+}
+
 class RemoteDependency {
   type: "remote"