[FEATURE] document recommendation for java package & maven group name

Is your feature request related to a problem?

the current template uses org.opensearch for the java package and maven artefacts. some (most? all?) third parties (examples: 1, 2) now release plugins also with these coordinates, making it unclear which of these are "official" plugins and which are not.

What solution would you like?

usually artefacts are published under the identifier of the actual publisher (e.g. "Example Inc." might publish as org.example instead of org.opensearch).
the outcome of this issue should be two things:

• decision whether 3rd parties should/can publish plugins also as org.opensearch
• documentation of this (either with a clear recommendation or clear discouragement) for new plugins as well as for existing plugins (most likely for these the statement is to not change anything or only change something when going to OpenSearch 3.0)

What alternatives have you considered?

in the current state where it's not documented and the template is using org.opensearch most will just use that and not think too much about it. thus it's a de-facto standard but not a de-jure standard.

Do you have any additional context?

https://github.com/opensearch-project/.github/issues/209 discusses the process to move repos to the opensearch org. if plugins start out in another organisation (or user account) and use another java package & maven group name they might have to be renamed, leading to a breaking change for their users (though again, this could be put off until the next major release, and when the plugin release is decoupled from the OpenSearch release this should become easier; see also https://github.com/opensearch-project/opensearch-plugin-template-java/issues/8#issuecomment-2331824622).