[opensearch] Enable Custom Dashboards via Declarative Config

[joluc]

Priority

(Low) Something is a little off

Context

Extend OpenSearch to provide a mechanism for deploying custom dashboards using a configuration-based approach. This could be implemented through methods such as:

• Utilizing a ConfigMap to store and manage custom dashboards.
• Introducing a Custom Resource (via operator) to define dashboards declaratively.

This feature would ensure consistent dashboard deployment across all OpenSearch installations and regions. It mirrors practices like for Plutono dashboards.