Add env.js to store global property, add legacy mode to support rest APIs backward compatibility, and update documents

[hxiao608]

Signoff by: Harold Xiao haoruxia@amazon.com

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Describe the solution you are proposing Add a new js file for setting and getting global properties, introduce a legacy flag to enable perfTop to run in a legacy mode, which will send out API request to opendistro endpoint

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

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Test Local build pass and succesfully test the new endpoint with the sample metrics command: ./build/opensearch-perf-top-macos --dashboard ${Dashboard} --endpoint localhost:9600 results:

Check List

• [x] New functionality includes testing.
  • [x] All tests pass
• [ ] New functionality has been documented.
  • [ ] New functionality has javadoc added
• [x] Commits are signed per the DCO using --signoff

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[sruti1312]

Since we dont have automated testing. Can we manually test the artifact and confirm everything is working as expected? Please add the manual testing details as well to the PR. Thanks!

[hxiao608]

Perftop can now render metrics with the new _plugins endpoint. The legacy mode will be able to render metrics after the setting up the Rest API BC for original _opendistro endpoint .