Add support for customizing HTTP headers (#1196)

This PR adds support for custom HTTP headers, introducing a
`--http-header` CLI flag to accept `key=value` pairs. These headers can
also be specified within the `setting.pkl` file.

Closes #633

SPICE: https://github.com/apple/pkl-evolution/pull/24

---------

Co-authored-by: Jen Basch <jbasch94@gmail.com>
Co-authored-by: Islon Scherer <islonscherer@gmail.com>

pkl-gradle/src/main/java/org/pkl/gradle/task/BasePklTask.java

@@ -48,6 +48,7 @@ import org.gradle.api.tasks.PathSensitivity;
 import org.gradle.api.tasks.TaskAction;
 import org.jspecify.annotations.Nullable;
 import org.pkl.commons.cli.CliBaseOptions;
+import org.pkl.core.Pair;
 import org.pkl.core.evaluatorSettings.Color;
 import org.pkl.gradle.utils.PluginUtils;
 
@@ -161,6 +162,10 @@ public abstract class BasePklTask extends DefaultTask {
   @Optional
   public abstract MapProperty<URI, URI> getHttpRewrites();
 
+  @Input
+  @Optional
+  public abstract ListProperty<Pair<Pattern, List<Pair<String, String>>>> getHttpHeaders();
+
   @Input
   @Optional
   public abstract Property<Boolean> getPowerAssertions();
@@ -218,6 +223,7 @@ public abstract class BasePklTask extends DefaultTask {
         getHttpProxy().getOrNull(),
         getHttpNoProxy().getOrElse(List.of()),
         getHttpRewrites().getOrNull(),
+        getHttpHeaders().getOrNull(),
         Map.of(),
         Map.of(),
         null,

pkl-gradle/src/main/java/org/pkl/gradle/task/ModulesTask.java

@@ -164,6 +164,7 @@ public abstract class ModulesTask extends BasePklTask {
         null,
         List.of(),
         getHttpRewrites().getOrNull(),
+        getHttpHeaders().getOrNull(),
         Map.of(),
         Map.of(),
         null,