[PROPOSAL] Document adding new plugins to the OpenSearch distribution

[dblock]

What/Why

What are you proposing?

Document the steps that need to be taken to add an OpenSearch plugin into the distribution, including experimental plugins.

What users have asked for this feature?

There seems to be some confusion on experimental plugins mainly around the code / usage / testing perspective. What flag needs to be added, how to add it, how can it be tested, what level of testing is required etc.

What problems are you trying to solve?

It should be easy to a developer to bring to life a new plugin that is part of the distribution.

What is the developer experience going to be?

New section in https://github.com/opensearch-project/opensearch-plugins/blob/main/BUILDING.md.

Are there any security considerations?

We need to document whether, for example, a security review is required for experimental plugins.

[bbarani]

We should also tag the features on our roadmap with "Experimental" flag to distinguish those features from GA feature scheduled for specific version.