Get aggregate code coverage, add coverage upload in test workflow and add badges in README

[tlfeng]

Issue #, if available:

Description of changes:

• Merge the code coverage of the root project and sub-project (Job Scheduler SPI) by utilizing the Gradle script in alerting repo: https://github.com/opendistro-for-elasticsearch/alerting/blob/master/build-tools/merged-coverage.gradle
• Use Codecov to visualize the code coverage result
• Add Coverage Report Upload action in test-and- build-workflow
• Add several badges in README
• Add codecov config file (TODO)

This PR carries the code coverage visualization that already in index-management and ‘alerting’ repo to job-scheduler
opendistro-for-elasticsearch/index-management#230
opendistro-for-elasticsearch/index-management#231
opendistro-for-elasticsearch/index-management#232
opendistro-for-elasticsearch/index-management#251
opendistro-for-elasticsearch/alerting#223
opendistro-for-elasticsearch/alerting#231

Concern: I think we should discuss which class is needed to do the code coverage analysis, and which don't.

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

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@2aaae0e). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #88   +/-   ##
=========================================
  Coverage          ?   43.93%           
  Complexity        ?      118           
=========================================
  Files             ?       15           
  Lines             ?      858           
  Branches          ?       89           
=========================================
  Hits              ?      377           
  Misses            ?      454           
  Partials          ?       27           

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