Make NamedXContent available to extensions

[dbwiddis]

Description

Implements the getNamedXContent() extension point:

1. Adds getNamedXContent() to the Extension interface, so that the ExtensionsRunner can read it. This will allow direct use of existing plugin method of this name.
2. Creates an ExtensnionNamedXContentRegistry which combines the Extension's custom content and the core XContent from OpenSearch. Getters fetch either the combined registry (needed as an input for various parsers) or just the custom content (presently unused, but potentially available for transport request/response (no need, functionality already on ExtensionsRunner)
3. Exposes the ExtensionNamedXContentRegistry via a getter to extensions so they can easily integrate them into REST Handlers by:
   • Passing the extensionsRunner object from the Extension (inherited from BaseExtension) to the RestHandler
   • Using extensionsRunner.getNamedXContentRegistry().getRegistry() to return the (combined) registry to pass to parsers

See AD #725 for an implementation leveraging these improvements!

Issues Resolved

Fixes #208
Fixes #131
Fixes #94

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[codecov-commenter]

Codecov Report

Merging #244 (cbf203f) into main (6df1fa4) will increase coverage by 3.65%.
The diff coverage is 100.00%.

❗ Current head cbf203f differs from pull request most recent head 1a83f71. Consider uploading reports for the commit 1a83f71 to get more accurate results

@@             Coverage Diff              @@
##               main     #244      +/-   ##
============================================
+ Coverage     67.19%   70.84%   +3.65%     
- Complexity      102      115      +13     
============================================
  Files            24       25       +1     
  Lines           506      542      +36     
  Branches         17       17              
============================================
+ Hits            340      384      +44     
+ Misses          154      146       -8     
  Partials         12       12              

Impacted Files	Coverage Δ
...rc/main/java/org/opensearch/sdk/BaseExtension.java	77.77% <100.00%> (+2.77%)	⬆️
src/main/java/org/opensearch/sdk/Extension.java	63.63% <100.00%> (+3.63%)	⬆️
...opensearch/sdk/ExtensionNamedXContentRegistry.java	100.00% <100.00%> (ø)
...main/java/org/opensearch/sdk/ExtensionsRunner.java	71.05% <100.00%> (+5.86%)	⬆️
...rch/sdk/handlers/ExtensionsInitRequestHandler.java	100.00% <100.00%> (ø)
...k/handlers/EnvironmentSettingsResponseHandler.java	64.70% <0.00%> (+29.41%)	⬆️

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[owaiskazi19]

@dbwiddis if we can add a design for getNamedXContent() in DESIGN.md would be great.

[owaiskazi19]

#94 talks about retry for handling transport errors by doing a retry. Are we handling it in this PR?

[dbwiddis]

@dbwiddis if we can add a design for getNamedXContent() in DESIGN.md would be great.

Sure, I'll provide one. It's a good template for other similar extension points.

https://i.imgflip.com/70rp2r.jpg

[dbwiddis]

#94 talks about retry for handling transport errors by doing a retry. Are we handling it in this PR?

No, but #174 addresses this.