add ssl bundle support for autoconfigured opensearch client

[aivish]

Description

Allows to configure SSL Bundle using opensearch.restclient.ssl.bundle.

Note: the certificate in integration tests is from opensearch security plugin's demo config (https://github.com/opensearch-project/security/blob/main/src/main/java/org/opensearch/security/tools/democonfig/Certificates.java#L157), if the certificate is changed in a future version then the integration test would probably break.

Looked at the SSL hot reload, but implementing it doesn't look that straight forward:

• Saw some simple examples where a singleton bean is deleted and registered with DefaultSingletonBeanRegistry (could re-create the RestClient bean), but that doesn't really work because it would not effect the already autowired/injected beans.
• Seems like this also requires changes on RestClient side, so it's possible to keep the same instance/bean and update its underlying http client. For example, allow swapping out the Apache HTTP client instance with a new one (currently it's final) or maybe the Apache HTTP client allows to update SSLContext, either way need some method on RestClient side that allows something like that.

Where should I document that hot reload is not supported? The documentation for available properties is in javadoc, but I can't add anything to the restclient.ssl.bundle because the class is from Elasticsearch. Thought about adding Restclient to OpenSearchProperties, but that would be breaking change in projects that do something like ElasticsearchProperties.Restclient restclientProperties = openSearchProperties.getRestclient().

Issues Resolved

resolve #396

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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[reta]

Thanks a lot @aivish LGTM but some tests need adjustments

[aivish]

should be better now, opensearch 1.3.x uses different certificate, had to add it to the trusted certs as well

if the security plugin will change the demo certificate often then could get a bit annoying, but hopefully the test won't cause too much trouble for you..