Add Destination converter for Notification Channels and utility class to call Notification API

[lezzago]

Issue #, if available:
#105
Description of changes:

• Add Destination converter for Notification Channels
• Add utility class to call Notification API
  CheckList:
  [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.
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[codecov-commenter]

Codecov Report

Merging #137 (1c45a2c) into main (ff57c54) will decrease coverage by 1.64%.
The diff coverage is 58.92%.

@@             Coverage Diff              @@
##               main     #137      +/-   ##
============================================
- Coverage     80.46%   78.82%   -1.65%     
+ Complexity      215      214       -1     
============================================
  Files           154      158       +4     
  Lines          5873     6338     +465     
  Branches        706      755      +49     
============================================
+ Hits           4726     4996     +270     
- Misses          768      932     +164     
- Partials        379      410      +31     

Impacted Files	Coverage Δ
...g/opensearch/alerting/util/NotificationAPIUtils.kt	0.00% <0.00%> (ø)
...pensearch/alerting/elasticapi/ElasticExtensions.kt	42.85% <0.00%> (-9.07%)	⬇️
...ng/actionconverter/EmailAccountActionsConverter.kt	76.47% <76.47%> (ø)
...ing/actionconverter/DestinationActionsConverter.kt	80.66% <80.66%> (ø)
...ting/actionconverter/EmailGroupActionsConverter.kt	86.44% <86.44%> (ø)
...ing/destination/client/DestinationEmailClient.java	72.50% <0.00%> (-5.00%)	⬇️
.../kotlin/org/opensearch/alerting/util/IndexUtils.kt	72.09% <0.00%> (-2.33%)	⬇️
.../kotlin/org/opensearch/alerting/core/JobSweeper.kt	71.72% <0.00%> (-0.53%)	⬇️
...ing/model/destination/DestinationContextFactory.kt	66.66% <0.00%> (ø)
... and 3 more

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[adityaj1107]

Can we mark this as internal as well, Since it is also being used inside another function?