Update REST resources to follow new OpenSearch naming convention

[jotok]

Plugin resources will appear under /_plugins instead of /_opendistro.

Describe the solution you are proposing
Following the upgrade guide to rename our REST resources.

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

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov-commenter]

Codecov Report

Merging #14 (ca460ff) into main (67d4ab7) will increase coverage by 0.09%.
The diff coverage is 80.76%.

@@             Coverage Diff              @@
##               main      #14      +/-   ##
============================================
+ Coverage     72.12%   72.21%   +0.09%     
- Complexity      339      348       +9     
============================================
  Files            43       44       +1     
  Lines          2461     2473      +12     
  Branches        150      155       +5     
============================================
+ Hits           1775     1786      +11     
- Misses          582      584       +2     
+ Partials        104      103       -1     

Impacted Files	Coverage Δ	Complexity Δ
...ormanceanalyzer/http_action/config/RestConfig.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...on/config/PerformanceAnalyzerResourceProvider.java	36.84% <60.00%> (+0.17%)	9.00 <0.00> (+1.00)
...config/PerformanceAnalyzerClusterConfigAction.java	86.36% <100.00%> (+0.31%)	10.00 <2.00> (+1.00)
...action/config/PerformanceAnalyzerConfigAction.java	92.20% <100.00%> (+0.42%)	24.00 <2.00> (+4.00)
...rformanceAnalyzerOverridesClusterConfigAction.java	80.95% <100.00%> (+0.22%)	18.00 <2.00> (+1.00)
...ormanceanalyzer/writer/EventLogQueueProcessor.java	63.33% <0.00%> (+5.00%)	8.00% <0.00%> (+2.00%)

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[dblock]

These LEGACY_PA_BASE_URI should really be LEGACY_OPENDISTRO_PA_BASE_URI. We weren't super clear in the doc, I fixed it in opensearch-project/opensearch-plugins#33.