Use checkstyle for javadoc checks

[dbwiddis]

Description

Replaces the existing combination of RequireJavadoc and the JDK's doclint with much more configurable javadoc checking using Gradle's checkstyle plugin. RequireJavadoc lost a lot of its usefulness with doclint changes in JDK 18.

Changes:

• Uses MissingJavadocType module to require javadocs for public and protected classes (including interfaces and enums)
• Uses MissingJavadocMethod module to require javadocs for public and protected methods, excluding getters and setters
• Uses JavadocMethod module with two configurations. Requires @param and @return for all methods. Allows excluding @return for the various extension interfaces where we've copied the plugin documentation.
• Uses the AvoidStarImport module to replace the custom version we hacked into spotless checks (but which it couldn't auto-fix)

Initial commit intentionally failing checks to demonstrate the failures it caught. Second commit fixes those.

Issues Resolved

Fixes #496
Re-implements fix for #3 and #27

See comment thread on #18 starting here regarding use of checkstyle. The removal from OpenSearch was due to conflicts in formatting configurations, but has no impact on the additional capabilities checkstyle brings to fill gaps where spotless (and any other tools we have tried) have failed.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[codecov-commenter]

Codecov Report

Merging #551 (f9e7457) into main (0da23e1) will not change coverage.
The diff coverage is n/a.

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@@            Coverage Diff            @@
##               main     #551   +/-   ##
=========================================
  Coverage     67.67%   67.67%           
  Complexity      200      200           
=========================================
  Files            36       36           
  Lines           863      863           
  Branches         26       26           
=========================================
  Hits            584      584           
  Misses          269      269           
  Partials         10       10           

Impacted Files	Coverage Δ
.../main/java/org/opensearch/sdk/ActionExtension.java	57.14% <ø> (ø)
...a/org/opensearch/sdk/BaseExtensionRestHandler.java	88.00% <ø> (ø)
.../main/java/org/opensearch/sdk/EngineExtension.java	100.00% <ø> (ø)
src/main/java/org/opensearch/sdk/SDKClient.java	90.90% <ø> (ø)
.../main/java/org/opensearch/sdk/SearchExtension.java	15.09% <ø> (ø)

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[owaiskazi19]

Overall LGTM! One suggestion. Thanks @dbwiddis for bringing this in. I really think Spotless should consider having these checks though. 😉