Fix py38 aync agent service and add async agent test

[Future-Outlier]

TL;DR

As title.

Type

• Bug Fix
• Feature
• Plugin

Are all requirements met?

• Code completed
• Smoke tested
• Unit tests added
• Code documentation added
• Any pending items have an associated Issue

Complete description

You can refer to this pr.
#1848

Tracking Issue

#1848
flyteorg/flyte#3936

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (11e98fb) 50.38% compared to head (67e4838) 54.76%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1866      +/-   ##
==========================================
+ Coverage   50.38%   54.76%   +4.37%     
==========================================
  Files         248      292      +44     
  Lines       19884    22076    +2192     
  Branches     3356     2172    -1184     
==========================================
+ Hits        10019    12090    +2071     
- Misses       9702     9837     +135     
+ Partials      163      149      -14     

Files	Coverage Δ
flytekit/extend/backend/agent_service.py	0.00% <0.00%> (ø)

... and 70 files with indirect coverage changes

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