Refactored code to account for notifications server features API change

[amsiglan]

Description

Notifications dashboards server route for fetching features (/api/notifications/features) was updated in PR https://github.com/opensearch-project/dashboards-notifications/pull/181 so that we can fetch the channel types for which the channels should be shown in options menu. This PR makes the required code changes in alerting to consume these updates in notification.

NOTE: The changes in the dashboards-notifications PR 181 are required for the changes in this PR, else it will break alerting.

Issues Resolved

#795

Check List

• [ ] New functionality includes testing.
  • [ ] All tests pass
• [ ] New functionality has been documented.
  • [ ] New functionality has javadoc added
• [x] 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 48.67%. Comparing base (413f1de) to head (7e13d2d). Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #916      +/-   ##
==========================================
- Coverage   48.69%   48.67%   -0.02%     
==========================================
  Files         236      236              
  Lines        6845     6847       +2     
  Branches      996      996              
==========================================
  Hits         3333     3333              
- Misses       3509     3511       +2     
  Partials        3        3              

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[opensearch-trigger-bot[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:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/alerting-dashboards-plugin/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/alerting-dashboards-plugin/backport-2.x
# Create a new branch
git switch --create backport-916-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 c239ded7f5fe94984fedae7fae63157503bb8099
# Push it to GitHub
git push --set-upstream origin backport-916-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/alerting-dashboards-plugin/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport-916-to-2.x.