feat: Session lifecycle event callback as webhook

[achimnol]

refs lablup/backend.ai#362

[codecov]

Codecov Report

Merging #567 (15b4ae3) into main (0388caf) will decrease coverage by 0.04%.
The diff coverage is 46.15%.

@@            Coverage Diff             @@
##             main     #567      +/-   ##
==========================================
- Coverage   48.81%   48.76%   -0.05%     
==========================================
  Files          53       53              
  Lines        8999     9012      +13     
==========================================
+ Hits         4393     4395       +2     
- Misses       4606     4617      +11     

Impacted Files	Coverage Δ
src/ai/backend/manager/models/kernel.py	44.60% <ø> (ø)
src/ai/backend/manager/registry.py	18.03% <ø> (ø)
src/ai/backend/manager/server.py	59.25% <ø> (-0.25%)	⬇️
src/ai/backend/manager/models/base.py	54.09% <46.15%> (-0.95%)	⬇️

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[adrysn]

The webhook request is successfully received by the MLOps pipeline server. However, the session event order is not guaranteed. For example, in the following Manager log, the started event is sent after the terminated event:

[inureyes]

LGTM (bugfix part)