Support declarative plugin management starting Elasticsearch 7.16

[sebgl]

Elasticsearch supports declarative plugin management starting 7.16: elastic/elasticsearch#70219, through an elasticsearch-plugins.yml file (official docs).

I think it would make sense for us to either:

• document how to rely on that feature by mounting a configmap through the podTemplate spec, rather than documenting our init container method
• expose that feature in the Elasticsearch CRD (and document it)

[thbkrkr]

Relates to #3530.

[pebrc]

If we wanted to support declarative plugin installation in the CRD we would have to find a solution for older versions of Elasticsearch pre 7.16.

• we could validate that the version in spec is at least 7.16 when a users specifies plugins
• we could fall back to init container based installation for earlier versions

More fundamentally, however, I wonder if we really should support this feature first class in the CRD as it encourages to use a sub-optimal way of installing the plugins (at least on K8s if I understand correctly how the new feature works we still have the problem that plugin installation will need to be repeated on each Pod recreation). A custom Docker image still sidesteps the potential issues of repeated installation and avoids installation failures due to network issues.

So maybe adjusting our documentation to illustrate the declarative approach instead would be preferable?

[needleshaped]

Relates to elastic/elasticsearch#55443