Make notifications optional

[amsiglan]

Description

Forward porting changes from #770 to keep Notifications optional in detector creation workflow since there is a possibility that Notification backend plugin exists but not the frontend plugin in which case the user won't be able to create a channel and thus cannot create a detector

Check List

• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b3ddb58) 31.14% compared to head (c2d74d2) 31.18%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #796      +/-   ##
==========================================
+ Coverage   31.14%   31.18%   +0.03%     
==========================================
  Files         151      151              
  Lines        5005     4999       -6     
  Branches      857      855       -2     
==========================================
  Hits         1559     1559              
+ Misses       3255     3250       -5     
+ Partials      191      190       -1     

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[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.x 2.x
# Navigate to the new working tree
cd .worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-796-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 5bf43a624f4563d53120c277773972722c3fa9f2
# Push it to GitHub
git push --set-upstream origin backport/backport-796-to-2.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-796-to-2.x.