[Enhancement] Fetch system index mappings from json file instead of string constants

[pyek-bot]

Description

Moves the index mappings for system indices into json files and fetches the version from the file itself.
Version should be provided under "_meta"."schema_version"

All mappings were fetched by using the below method in MlIndex.java

public static void main(String[] args) {
        for (MLIndex index : values()) {
            System.out.println(index.getIndexName() + "\n"
                    + index.getMapping() + "\n");
        }
    }

Related Issues

Resolves #2951

Enhancements

• Add a required_fields object under _meta object to validate if these fields are present in the mapping
• Support placeholders to enable reusability

These enhancements will be raised in a separate PR: https://github.com/pyek-bot/ml-commons/pull/1/files

Testing

• Mappings were compared by fetching the existing mappings from main and comparing against new mappings using the above mentioned code block
• Added validation on mappings to ensure they follow a particular schema
• Added a test case that initializes the enum so that any failures in mapping creation can be caught before runtime
  https://github.com/pyek-bot/ml-commons/pull/1/files
• Added UTs for helper methods
• Ran integration tests to ensure all indices are created
• Manually created all system indices, fetched them via the GET api and validated the mappings

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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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[pyek-bot]

@austintlee @HenryL27 I have also modified this class to use the MLIndex enum, please review the same. I didn't want to do a full refactor for now as it may impact functionality. Added as a todo on top of the class.

[dhrubo-os]

Let's add more details about how do you test this feature in the PR description?

[dhrubo-os]

IndexUtilsTest > testGetMappingFromFile FAILED
    org.junit.ComparisonFailure at IndexUtilsTest.java:71
    ```

[pyek-bot]

IndexUtilsTest > testGetMappingFromFile FAILED
    org.junit.ComparisonFailure at IndexUtilsTest.java:71
    ```

On it

Edit: Fixed - issue was due to eol characters being different on different OS

[pyek-bot]

@Zhangxunmt @dhrubo-os please re-review when you get the time

I am looking at the test failures, seems to be flaky tests