Always initialize SDKActionModule

[dbwiddis]

Description

Improves on #520

• Always initializes the SDKActionModule, handling internals with sensible (empty) defaults
• Removes (no longer necessary) null checks and Nullable annotations
• Addresses this comment by simply returning action not registered exception, removing separate exception handling for nulls.
• BaseExtension no longer implements ActionExtension

Issues Resolved

Partial implementation of #525

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[codecov-commenter]

Codecov Report

Merging #537 (57f888d) into main (30759d2) will increase coverage by 0.15%.
The diff coverage is 100.00%.

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@@             Coverage Diff              @@
##               main     #537      +/-   ##
============================================
+ Coverage     67.67%   67.83%   +0.15%     
- Complexity      197      198       +1     
============================================
  Files            34       34              
  Lines           857      858       +1     
  Branches         29       26       -3     
============================================
+ Hits            580      582       +2     
- Misses          264      266       +2     
+ Partials         13       10       -3     

Impacted Files	Coverage Δ
...rc/main/java/org/opensearch/sdk/BaseExtension.java	84.61% <ø> (ø)
...rch/sdk/sample/helloworld/HelloWorldExtension.java	66.66% <ø> (ø)
...main/java/org/opensearch/sdk/ExtensionsRunner.java	72.06% <100.00%> (+0.32%)	⬆️
src/main/java/org/opensearch/sdk/SDKClient.java	90.90% <100.00%> (-2.12%)	⬇️
...ava/org/opensearch/sdk/action/SDKActionModule.java	91.66% <100.00%> (+1.34%)	⬆️
...rch/sdk/handlers/ExtensionsInitRequestHandler.java	100.00% <100.00%> (+4.00%)	⬆️

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