[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.resolved

@@ -1,5 +1,5 @@
 {
-  "originHash" : "488bc8ba375dd6a42de3f34d4290056a146f9bd52f1bd6cd8ed81cbf4c63984f",
+  "originHash" : "50e672f8971a68675dcb3ca27b4bd4832c7e96f2216ae062f002554d445c6714",
   "pins" : [
     {
       "identity" : "lcecryptokitbinary",