Move the index mapping definitions to standalone json files

[arsen-es]

All mappings are currently defined in src/test/java/com/amazon/opendistroforelasticsearch/sql/esintgtest/TestUtils.java as java strings, which makes it difficult to find/maintain/properly indent when updating/verify validity of json. We need to create one small json per index mapping and load it at run-time.

[pakio]

@arsen-es Hi! I tried to fix this issue. if you haven't picked up yet, i'd be grateful if you would check my PR #526

[anirudha]

We Will prioritize it next week , thanks!

[dai-chen]

@pakio Thanks for your fix! Resolving the issue.