Fix repository-s3 plugin to work with s3-compatible blob stores

[cwperks]

Description

This PR modifies the S3 builder to only verify checksum calculation and verification when required.

final S3ClientBuilder builder = S3Client.builder()
            // added
            .requestChecksumCalculation(RequestChecksumCalculation.WHEN_REQUIRED)
            .responseChecksumValidation(ResponseChecksumValidation.WHEN_REQUIRED)
            .addPlugin(LegacyMd5Plugin.create());

Related Issues

Resolves opensearch-project#18240

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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