Adds developer certificate of origin check

[downsrob]

Signed-off-by: Robert Downs downsrob@amazon.com

Issue #, if available:
#156

Description of changes:
Adds developer certificate of origin check to github workflows. The mentioned issue also requires updating the repository's contributing guide, but as the Index Management contributing guide simply links to the OpenSearch guide, this is not necessary.

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 #189 (57f6c5c) into main (bdc1f9e) will decrease coverage by 0.13%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main     #189      +/-   ##
============================================
- Coverage     74.80%   74.67%   -0.14%     
- Complexity     1896     1897       +1     
============================================
  Files           260      260              
  Lines         11094    11094              
  Branches       1746     1746              
============================================
- Hits           8299     8284      -15     
- Misses         1856     1875      +19     
+ Partials        939      935       -4     

Impacted Files	Coverage Δ
...statemanagement/model/destination/CustomWebhook.kt	65.21% <0.00%> (-27.54%)	⬇️
...arch/indexmanagement/rollup/RollupSearchService.kt	55.55% <0.00%> (-3.71%)	⬇️
.../rollup/action/start/TransportStartRollupAction.kt	66.26% <0.00%> (-2.41%)	⬇️
.../opensearch/indexmanagement/rollup/model/Rollup.kt	84.43% <0.00%> (ø)
...nsform/action/stop/TransportStopTransformAction.kt	72.15% <0.00%> (ø)
...anagement/indexstatemanagement/model/Transition.kt	67.69% <0.00%> (+3.07%)	⬆️
...dexmanagement/transform/model/TransformMetadata.kt	89.15% <0.00%> (+3.61%)	⬆️
...nt/indexstatemanagement/model/destination/Chime.kt	54.54% <0.00%> (+13.63%)	⬆️

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