Added support for OpenSearch Serverless.

[dblock]

Signed-off-by: dblock dblock@amazon.com

• Adds x-amz-content-sha256 which is supported by both Managed OpenSearch and OpenSearch Serverless, replaces #90, closes #88.
• Adds service name to command-line options.
• Bumps OpenSearch test cluster version to 2.3, closes #93.
• Bumps version to 2.3.0 since we're adding a feature.
• Tested with both OpenSearch 2.3 and Serverless, with make run_sample and make run_v5_sample.

Similar to https://github.com/opensearch-project/opensearch-java/pull/339/files.

Pull Request Checklist:

• [x] I have added a contribution line at the top of CHANGELOG.md.

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

[dblock]

Want (me) to do a release @acm19?

[acm19]

@dblock Please do, I'll take a look at the manual on Maven Central tomorrow. I need to find some time to change that.

[dblock]

Did.

Can you help me get access to the maven release UX as well? I cannot login to https://s01.oss.sonatype.org (I can login to https://oss.sonatype.org), and don't see this package in any lists. I think this needs a JIRA ticket or something? My username is dblock / dblock at dblock dot org everywhere.

[acm19]

@dblock I've requested it already: https://issues.sonatype.org/browse/OSSRH-88283. If they take too long I'll do the publishing myself this time, I'll wait a couple of days.

[dblock]

Got access, thanks. Closed the repository at https://s01.oss.sonatype.org/#stagingRepositories and hit release.