chore: change sequencer_node_handle type

[nadin-Starkware]

Stack:

• feat: add test case for remote connection flow #2647
• chore: add mock_system_mode to create config function #2646
• chore: return vector of SequencerNodeConfig in create config function #2638
• chore: add get_http_server_config function #2637
• chore: change sequencer_node_handle type #2636 ⬅
• chore: add serial_test crate #2635
• chore: add mock system mode enum #2632

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.

[reviewable-StarkWare]

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@                 Coverage Diff                  @@
##             spr/main/fa47beb6    #2636   +/-   ##
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  Coverage                     ?   35.00%           
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  Files                        ?      277           
  Lines                        ?    32227           
  Branches                     ?    32227           
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  Hits                         ?    11282           
  Misses                       ?    19938           
  Partials                     ?     1007           

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