Feature/conversation backport to 2.x

[HenryL27]

Description

Bring RAG pipeline and Conversation memory API into 2.x

Issues Resolved

#1150

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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[HenryL27]

#1288 will fix jarhell (should I just add those changes here independently too?)

[codecov]

Codecov Report

Merging #1286 (b670854) into 2.x (86eb953) will increase coverage by 0.49%.
The diff coverage is 94.01%.

@@             Coverage Diff              @@
##                2.x    #1286      +/-   ##
============================================
+ Coverage     78.86%   79.36%   +0.49%     
- Complexity     2142     2246     +104     
============================================
  Files           168      188      +20     
  Lines          8741     9074     +333     
  Branches        877      903      +26     
============================================
+ Hits           6894     7202     +308     
- Misses         1446     1464      +18     
- Partials        401      408       +7     

Flag	Coverage Δ
ml-commons	79.36% <94.01%> (+0.49%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...nerative/client/MachineLearningInternalClient.java	72.41% <72.41%> (ø)
...ing/generative/GenerativeQAProcessorConstants.java	75.00% <75.00%> (ø)
...rg/opensearch/ml/plugin/MachineLearningPlugin.java	96.18% <84.61%> (-2.88%)	⬇️
...nswering/generative/ext/GenerativeQAParamUtil.java	88.88% <88.88%> (ø)
...ring/generative/GenerativeQAResponseProcessor.java	91.07% <91.07%> (ø)
.../generative/client/ConversationalMemoryClient.java	96.87% <96.87%> (ø)
...ch/ml/rest/RestMemoryCreateConversationAction.java	100.00% <100.00%> (ø)
...rch/ml/rest/RestMemoryCreateInteractionAction.java	100.00% <100.00%> (ø)
...ch/ml/rest/RestMemoryDeleteConversationAction.java	100.00% <100.00%> (ø)
...arch/ml/rest/RestMemoryGetConversationsAction.java	100.00% <100.00%> (ø)
... and 12 more

... and 2 files with indirect coverage changes

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