Mark HTTPClientRequest.Prepared as Sendable (#876)

`Transaction` is @unchecked Sendable, since its NIOLockedValueBox can
not infer Sendability if its value isn't Sendable. The @unchecked
Sendable annotation has hidden the fact, that
`HTTPClientRequest.Prepared` needs to be Sendable as well. Let's make
this right in this PR.

Author: fabianfett

Sources/AsyncHTTPClient/AsyncAwait/HTTPClientRequest+Prepared.swift

@@ -22,16 +22,16 @@ import struct Foundation.URL
 
 @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension HTTPClientRequest {
-    struct Prepared {
-        enum Body {
+    struct Prepared: Sendable {
+        enum Body: Sendable {
             case asyncSequence(
                 length: RequestBodyLength,
                 makeAsyncIterator: @Sendable () -> ((ByteBufferAllocator) async throws -> ByteBuffer?)
             )
             case sequence(
                 length: RequestBodyLength,
                 canBeConsumedMultipleTimes: Bool,
-                makeCompleteBody: (ByteBufferAllocator) -> ByteBuffer
+                makeCompleteBody: @Sendable (ByteBufferAllocator) -> ByteBuffer
             )
             case byteBuffer(ByteBuffer)
         }