Add BillingDependencies and shared implementation.

core-util/src/main/java/org/signal/core/util/billing/BillingDependencies.kt

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2024 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.core.util.billing
+
+import android.content.Context
+
+/**
+ * Provides a dependency model by which the billing api can request different resources.
+ */
+interface BillingDependencies {
+  /**
+   * Application context
+   */
+  val context: Context
+
+  /**
+   * Get the product id from the donations configuration object.
+   */
+  suspend fun getProductId(): String
+
+  /**
+   * Get the base plan id from the donations configuration object.
+   */
+  suspend fun getBasePlanId(): String
+}