Add WorkflowStep Factory and implement XContent-based Template Parsing

[dbwiddis]

Description

1. Creates a WorkflowStepFactory class to map a template "step type" to the correct instance
2. Parses the entire Use Case Template into XContent
3. Process the user inputs section for the workflow into WorkflowData params
4. Processes the user inputs section for each node into WorkflowData content
5. Refactors code to separate parsing functionality (WorkflowNode et. al.) from execution functionality (ProcessNode et. al.). Uses the WorfklowStep and Workflow Data when creating the ProcessNode.
6. Implements an executor service to use the OpenSearch thread pool for the async execution.
7. Added tests.
8. Fixed the ProcessNodeTests bug

Issues Resolved

Fixes #41
Fixes #42
Fixes #46
Fixes #50
Fixes #55

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[codecov]

Codecov Report

Merging #47 (2fca71b) into main (1d22bee) will increase coverage by 13.05%.
The diff coverage is 90.14%.

@@              Coverage Diff              @@
##               main      #47       +/-   ##
=============================================
+ Coverage     73.87%   86.93%   +13.05%     
- Complexity       65      163       +98     
=============================================
  Files             8       13        +5     
  Lines           245      574      +329     
  Branches         22       75       +53     
=============================================
+ Hits            181      499      +318     
+ Misses           54       52        -2     
- Partials         10       23       +13     

Files	Coverage Δ
...nsearch/flowframework/model/PipelineProcessor.java	100.00% <100.00%> (ø)
...a/org/opensearch/flowframework/model/Workflow.java	100.00% <100.00%> (ø)
...search/flowframework/workflow/CreateIndexStep.java	84.84% <100.00%> (ø)
...owframework/workflow/CreateIngestPipelineStep.java	90.62% <ø> (ø)
...opensearch/flowframework/workflow/ProcessNode.java	58.69% <88.88%> (ø)
.../opensearch/flowframework/FlowFrameworkPlugin.java	0.00% <0.00%> (ø)
...g/opensearch/flowframework/model/WorkflowEdge.java	84.84% <84.84%> (ø)
.../flowframework/workflow/WorkflowProcessSorter.java	92.85% <92.85%> (ø)
...g/opensearch/flowframework/model/WorkflowNode.java	90.54% <90.54%> (ø)
...a/org/opensearch/flowframework/model/Template.java	92.36% <92.36%> (ø)
... and 1 more

[dbwiddis]

Flipping back to draft to integrate the commits from #24 and #44 into the factory class (and delete my demo version).

[dbwiddis]

And flipping back to ready for review, after rebasing and also completing tasks from #42 and fixing #50.

[joshpalis]

initial pass, still need to go over the test classes

[dbwiddis]

To ease review, a general summary of the main part of this PR:

• Parsing. Starts with Template, which also parses workflow, which also parses WorkflowEdge and WorkflowNode.
• Process Sorting: takes the WorflowNodes and WorkflowEdges and creates the sorted ProcessNode sequence with links to predecessor ProcessNodes (no change from previous code, just separated out the parsing part)
• WorkflowStepFactory: takes the node "type" field and generates a unique instance of the correct workflow step

Additional changes:

• Made the sorter and step factory singletons taking the needed arguments that will go into the process nodes and workflow steps
• Passed an executor service around to have better control of threading. This can be changed in the future away from the generic one.
• Misc. cleanup of warnings (casting, deprecation)

[owaiskazi19]

Initial Pass! Thanks for adding XContent functionality to the project

[owaiskazi19]

Looks good overall with a minor comment/question.

[joshpalis]

Thanks @dbwiddis for adding the Pipeline Processor parsers!