verify client in download handler

[Hitenjain14]

Changes

Fixes

Tests

Tasks to complete before merging PR:

• Ensure system tests are passing. If not Run them manually to check for any regressions 📋
• Do any new system tests need added to test this change? do any existing system tests need updated? If so create a PR at 0chain/system_test
• Merge your system tests PR to master AFTER merging this PR

Associated PRs (Link as appropriate):

• 0chain:
• gosdk: add signature in header gosdk#1378
• system_test: fix client sig system_test#995
• zboxcli:
• zwalletcli:
• Other: ...

[codecov-commenter]

Codecov Report

Attention: 28 lines in your changes are missing coverage. Please review.

Comparison is base (cce361c) 14.40% compared to head (9f54632) 14.46%.

Files	Patch %	Lines
...et/blobbercore/handler/object_operation_handler.go	44.00%	11 Missing and 3 partials ⚠️
.../0chain.net/blobbercore/handler/storage_handler.go	55.17%	11 Missing and 2 partials ⚠️
code/go/0chain.net/blobbercore/handler/context.go	50.00%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           staging    #1378      +/-   ##
===========================================
+ Coverage    14.40%   14.46%   +0.06%     
===========================================
  Files          177      177              
  Lines        19859    19891      +32     
===========================================
+ Hits          2861     2878      +17     
- Misses       16642    16653      +11     
- Partials       356      360       +4     

Flag	Coverage Δ
Unit-Tests	14.46% <56.92%> (+0.06%)	⬆️

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[dabasov]

lgtm