Make FileChunks Sendable (#2871)

Motivation:

`FileChunks` is an `AsyncSequence` so should be `Sendable` otherwise
it's difficult to use.

Modifications:

- Conform `FileChunks` to `Sendable`

Result:

- Resolves #2752

Sources/NIOFileSystem/FileChunks.swift

@@ -20,7 +20,7 @@ import NIOPosix
 
 /// An `AsyncSequence` of ordered chunks read from a file.
 @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
-public struct FileChunks: AsyncSequence {
+public struct FileChunks: AsyncSequence, Sendable {
     enum ChunkRange {
         case entireFile
         case partial(Range<Int64>)