plugins/ism: Fix error notification types

[Jakob3xD]

Description

• Fix error notification types by making them pointers and adding omitempty
• Add test for channel
• Add missing struct for Channel field

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.99%. Comparing base (06a6dc8) to head (393ef12).
Report is 67 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #613       +/-   ##
===========================================
+ Coverage   57.29%   67.99%   +10.70%     
===========================================
  Files         315      376       +61     
  Lines        9823     8862      -961     
===========================================
+ Hits         5628     6026      +398     
+ Misses       2902     1555     -1347     
+ Partials     1293     1281       -12     

Flag	Coverage Δ
integration	60.35% <ø> (+9.51%)	⬆️
unit	15.37% <ø> (+2.53%)	⬆️

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

Files with missing lines	Coverage Δ
plugins/ism/api_policies.go	100.00% <ø> (ø)

... and 274 files with indirect coverage changes

[dblock]

Btw, I started looking at ISM APIs in the API spec in opensearch-project/opensearch-api-specification#568.