Add support for OpenSearch Serverless collections to the opensearch s…

[graytaylor0]

…ource

Description

This change adds a serverless flag in the opensearch source to support Amazon OpenSearch Serverless collections

aws:
  serverless: true

Serverless collections do not support point in time APIs, but I tested with both search_after and scroll and they worked as intended (the scroll deletion still fails from the client but is likely related to this issue (opensearch-project/opensearch-java#521)

Issues Resolved

Related to #1985

Check List

• New functionality includes testing.
• New functionality has a documentation issue. Please link to it in this PR.
  • New functionality has javadoc added
• Commits are signed with a real name per the DCO

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[dlvenable]

Is there a particular reason to remove this distribution?

[graytaylor0]

I removed it because it doesn't seem to exist. I think the distribution will be just like "elasticsearch"

[dlvenable]

It seems that ES 6.x and 7.x could be opendistro. But, ES versions beyond 7.10.2 would be elasticsearch only.

[graytaylor0]

I don't think elasticsearch or opendistro versions even publish their distribution. the root endpoint for elasticsearch does not return a distribution, and I think this was added for opensearch to differentiate it. I will download an opendistro and run it if I can to see what gets returned

[graytaylor0]

Confirmed that opendistro does not have a distribution returned, which is same behavior as elasticsearch. We are just hard-coding the distribution to be elasticsearch when no distribution is returned in the info() request.

curl -XGET https://localhost:9200 -u 'admin:admin' --insecure' --insecure
{
  "name" : "22654e359636",
  "cluster_name" : "docker-cluster",
  "cluster_uuid" : "rGtPyarcQ5-tJHR6TsSxJw",
  "version" : {
    "number" : "7.10.2",
    "build_flavor" : "oss",
    "build_type" : "tar",
    "build_hash" : "747e1cc71def077253878a59143c1f785afa92b9",
    "build_date" : "2021-01-13T00:42:12.435326Z",
    "build_snapshot" : false,
    "lucene_version" : "8.7.0",
    "minimum_wire_compatibility_version" : "6.8.0",
    "minimum_index_compatibility_version" : "6.0.0-beta1"
  },
  "tagline" : "You Know, for Search"
}