Send tar hash in the first BuildTransfer packet (#1149)

Send the hash of entire tar file in the first BuildTransfer packet to
prevent container-builder-shim from using stale cached contents.

This PR resolves #1143.
This PR relies on apple/container-builder-shim#64.

## Type of Change
- [X] Bug fix
- [ ] New feature  
- [ ] Breaking change
- [ ] Documentation update

## Motivation and Context
Current container-builder-shim uses only first few bytes of tar file as
checksum, which leads to the usage of stale cached contents if the
change of build context is not included in the first bytes of tar file.

## Testing
- [X] Tested locally
- [ ] Added/updated tests
- [ ] Added/updated docs

---------

Co-authored-by: Ronit Sabhaya <ronitsabhaya75@gmail.com>
Co-authored-by: J Logan <john_logan@apple.com>

Sources/ContainerBuild/BuildFSSync.swift

@@ -18,6 +18,7 @@ import Collections
 import ContainerAPIClient
 import ContainerizationArchive
 import ContainerizationOCI
+import CryptoKit
 import Foundation
 import GRPC
 
@@ -199,7 +200,7 @@ actor BuildFSSync: BuildPipelineHandler {
             format: .paxRestricted,
             filter: .none)
 
-        try Archiver.compress(
+        let tarHash = try Archiver.compress(
             source: contextDir,
             destination: tarURL,
             writerConfiguration: writerCfg
@@ -229,6 +230,25 @@ actor BuildFSSync: BuildPipelineHandler {
                 pathInArchive: URL(fileURLWithPath: rel))
         }
 
+        let hash = tarHash.compactMap { String(format: "%02x", $0) }.joined()
+        let header = BuildTransfer(
+            id: packet.id,
+            source: tarURL.path,
+            complete: false,
+            isDir: false,
+            metadata: [
+                "os": "linux",
+                "stage": "fssync",
+                "mode": "tar",
+                "hash": hash,
+            ]
+        )
+        var resp = ClientStream()
+        resp.buildID = buildID
+        resp.buildTransfer = header
+        resp.packetType = .buildTransfer(header)
+        sender.yield(resp)
+
         for try await chunk in try tarURL.bufferedCopyReader() {
             let part = BuildTransfer(
                 id: packet.id,