[api-upload-refactor] Add test target, relax decode constraints, add HTTPBody + MultipartForm

- Package: new LCEssentialsTests target (runs on iOS simulator)
- JSONDecoder.decode: T: Codable -> T: Decodable & Sendable (4 overloads);
  remove try! in decode(fromURL:); honour encoding param in decode(_:using:)
- Propagate & Sendable to Data.object, Dictionary.toObjetct, API.request return
- LCEHTTPBody: HTTPBody protocol, JSONBody, FormURLEncodedBody, RawBody, factories
- LCEMultipartForm: multipart builder, OutputStream-streamed serialize(),
  in-memory + on-disk file parts, ported mimeType(for:)
- Tests: StubURLProtocol harness + 15 tests

Package.swift

@@ -39,5 +39,8 @@ let package = Package(
         .target(
             name: "LCEssentials",
             dependencies: targetDependencies),
+        .testTarget(
+            name: "LCEssentialsTests",
+            dependencies: ["LCEssentials"]),
     ]
 )