Overriding defaultTitle with subject for SNS notifications

[uroskarama]

Issue #, if available: 578

Description of changes:
Using subject instead of defaultTitle if possible

CheckList:
[ ] 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-commenter]

Codecov Report

Merging #708 (fd9f12b) into main (817d9ef) will increase coverage by 0.20%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##               main     #708      +/-   ##
============================================
+ Coverage     76.11%   76.31%   +0.20%     
  Complexity      116      116              
============================================
  Files           124      124              
  Lines          6618     6619       +1     
  Branches        982      982              
============================================
+ Hits           5037     5051      +14     
+ Misses         1083     1067      -16     
- Partials        498      501       +3     

Impacted Files	Coverage Δ
.../util/destinationmigration/NotificationApiUtils.kt	57.14% <0.00%> (ø)
...ensearch/alerting/model/destination/Destination.kt	71.51% <0.00%> (+2.00%)	⬆️
.../kotlin/org/opensearch/alerting/util/IndexUtils.kt	72.91% <0.00%> (+2.08%)	⬆️
.../alerting/transport/TransportIndexMonitorAction.kt	62.57% <0.00%> (+2.63%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[opensearch-trigger-bot]

The backport to 2.5 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-2.5 2.5
# Navigate to the new working tree
cd .worktrees/backport-2.5
# Create a new branch
git switch --create backport/backport-708-to-2.5
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 88536b233c56d687ae61b7c31b69b883b7b1affe
# Push it to GitHub
git push --set-upstream origin backport/backport-708-to-2.5
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.5

Then, create a pull request where the base branch is 2.5 and the compare/head branch is backport/backport-708-to-2.5.