feat: Adds vault client

Sources/ConfigurationClient/ConfigurationClient.swift

@@ -59,6 +59,16 @@ extension ConfigurationClient: DependencyKey {
   public static var liveValue: Self {
     .live(environment: ProcessInfo.processInfo.environment)
   }
+
+  @_spi(Internal)
+  public static func mock(_ configuration: Configuration = .mock) -> Self {
+    .init(
+      find: { throw MockFindError() },
+      generate: Self().generate,
+      load: { _ in configuration },
+      write: Self().write
+    )
+  }
 }
 
 struct LiveConfigurationClient {
@@ -229,3 +239,5 @@ enum ConfigurationError: Error {
   case decodingError
   case fileExists(path: String)
 }
+
+struct MockFindError: Error {}