add checksum in download request to verify its consistency

[CormickKneey]

Description

1. add calculate_digest for() piece used in DownloadPieceResponse
2. rename calculate_file_hash() to calculate_file_digest()

Related Issue

dragonflyoss/dragonfly#3811

Motivation and Context

Part of Improvement of Security

[codecov]

Codecov Report

Attention: Patch coverage is 64.91228% with 20 lines in your changes missing coverage. Please review.

Project coverage is 33.18%. Comparing base (3339d5c) to head (17af9fd).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dragonfly-client/src/resource/piece_downloader.rs	0.00%	17 Missing ⚠️
dragonfly-client/src/grpc/dfdaemon_upload.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #979      +/-   ##
==========================================
+ Coverage   33.04%   33.18%   +0.14%     
==========================================
  Files          59       59              
  Lines        9681     9727      +46     
==========================================
+ Hits         3199     3228      +29     
- Misses       6482     6499      +17     

Files with missing lines	Coverage Δ
dragonfly-client-storage/src/metadata.rs	62.90% <100.00%> (+4.37%)	⬆️
dragonfly-client-util/src/digest/mod.rs	80.48% <100.00%> (ø)
dragonfly-client/src/grpc/dfdaemon_upload.rs	0.00% <0.00%> (ø)
dragonfly-client/src/resource/piece_downloader.rs	3.33% <0.00%> (-1.32%)	⬇️

[gaius-qi]

LGTM