feat: add last keep alive at tracking for scheduler service

[chlins]

This pull request introduces functionality to track the last heartbeat time of schedulers in the system. The changes include updates to the Scheduler model, modifications to RPC server methods for scheduler creation and updates, and enhancements to the keep-alive mechanism.

Model Updates:

• Added a new LastHeartbeat field of type time.Time to the Scheduler struct to store the last heartbeat timestamp. (manager/models/scheduler.go, manager/models/scheduler.goR39)
• Imported the time package to support the new field. (manager/models/scheduler.go, manager/models/scheduler.goR19-R20)

RPC Server Enhancements:

• Updated the UpdateScheduler and createScheduler methods to set the LastHeartbeat field to the current time when a scheduler is updated or created. (manager/rpcserver/manager_server_v2.go, [1] [2]
• Enhanced the KeepAlive method to update the LastHeartbeat field for schedulers upon successful keep-alive requests. (manager/rpcserver/manager_server_v2.go, manager/rpcserver/manager_server_v2.goR874-R887)
• Imported the time package to support the timestamp functionality in RPC server methods. (manager/rpcserver/manager_server_v2.go, manager/rpcserver/manager_server_v2.goR24)

Description

Related Issue

#4179

Motivation and Context

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.

Project coverage is 32.62%. Comparing base (88cc9b6) to head (b7e67a9).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
manager/rpcserver/manager_server_v2.go	0.00%	14 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4213   +/-   ##
=======================================
  Coverage   32.62%   32.62%           
=======================================
  Files         350      350           
  Lines       41204    41216   +12     
=======================================
+ Hits        13442    13447    +5     
- Misses      26871    26879    +8     
+ Partials      891      890    -1     

Flag	Coverage Δ
unittests	32.62% <0.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
manager/rpcserver/manager_server_v2.go	0.00% <0.00%> (ø)

... and 2 files with indirect coverage changes

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[gaius-qi]

LGTM

[Liam-Zhao]

LGTM

[Liam-Zhao]

LGTM

[gaius-qi]

LGTM