refactor(build): extract reusable tasks into custom Gradle classes

Replaced inlined implementations of `ForbiddenAuthorizationHeaderTask`, `FeatureIsolationTask`, and `BundleBudgetTask` with dedicated custom Gradle task classes for improved maintainability and reusability. Updated task registrations in `build.gradle.kts`.

buildSrc/build.gradle.kts

@@ -0,0 +1,11 @@
+plugins {
+    `kotlin-dsl`
+}
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    implementation(gradleApi())
+}