Support IF NOT EXISTS in create statement

[dai-chen]

Description

Add IF NOT EXISTS in create index statement. This is useful along with checkpoint_location option introduced in #39. We just submit the exact same job (create with if not exists statement) to Spark to restart index refreshing, instead of submitting new job with REFRESH statement.

Doc: https://github.com/dai-chen/opensearch-spark/blob/support-if-not-exists/docs/index.md

Issues Resolved

#27

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[vmmusings]

In general, what is preferred REFRESH INDEX statement or CREATE INDEX with IF NOT EXISTS.

[dai-chen]

In general, what is preferred REFRESH INDEX statement or CREATE INDEX with IF NOT EXISTS.

I think run REFRESH statement directly is more clear. CREATE INDEX with IF NOT EXISTS is for use case that we just want to restart with same job code without looking into it. Anyway, this difference are only in SQL layer. In either case, only FlintSpark.refreshIndex API will be triggered underlying. Thanks!